D028 Communicable Disease Outbreak Assessment Presentation

D028 Communicable Disease Outbreak Assessment Presentation

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 Western Governors University

D028 Advanced Health Assessment for Patients and Populations

Prof. Name

Date

Introduction

An epidemiologic outbreak investigation helps public health professionals identify cases, verify the disease, determine how an infection is spreading, measure the extent of illness, and implement measures to prevent additional cases. In the fictional Crab Apple Valley outbreak, investigators applied core epidemiologic methods to examine a localized outbreak of plague caused by Yersinia pestis. The investigation used a standardized case definition, surveillance, person-place-time analysis, epidemiologic calculations, an epidemic curve, and an observational study design.

The scenario suggests that the initial infections were associated with exposure to infected fleas in an environment with rodent activity. As some patients developed pneumonic plague, close contacts, including healthcare workers, faced potential exposure to infectious respiratory droplets. This created an opportunity for investigators to examine both the suspected initial vector-borne transmission and possible secondary person-to-person transmission.

The Crab Apple Valley outbreak demonstrates how epidemiology connects individual patient assessment with population-level disease prevention. Concepts such as case identification, active and passive surveillance, attack rates, cumulative incidence, case fatality rate, incubation periods, epidemic curves, outbreak classification, infection prevention, and risk communication all contribute to an effective outbreak response.

Background of the Crab Apple Valley Plague Outbreak

The Crab Apple Valley scenario describes a localized outbreak involving Yersinia pestis, the bacterium that causes plague. The initial patient, Jack Kalani, worked on an orchard farm where rodent activity created conditions associated with possible exposure to infected fleas.

After becoming ill, Jack developed symptoms consistent with plague and later developed pulmonary involvement. Once pneumonic plague was suspected, people who had close contact with him became important to the epidemiologic investigation.

Potentially exposed individuals included people who encountered Jack at the Crab Apple Valley farmers’ markets, Community Hospital, and other locations. Healthcare professionals and emergency medical personnel involved in his care or resuscitation were also evaluated because of their potential exposure.

Investigators used descriptive epidemiology to examine the outbreak according to person, place, and time. These findings helped generate hypotheses about where exposure occurred and whether subsequent cases could have resulted from person-to-person transmission.

Case Definition for the Crab Apple Valley Outbreak

A case definition is a standardized set of clinical, laboratory, and epidemiologic criteria used to determine whether a person should be counted as a case during an outbreak. Establishing the definition early helps investigators classify individuals consistently and maintain reliable case counts.

A comprehensive outbreak case definition may consider:

• Clinical criteria: Signs, symptoms, and examination findings.
• Laboratory criteria: Results from diagnostic or confirmatory testing.
• Epidemiologic criteria: Relevant exposure, contact, location, or timing.
• Case classification: Suspected, probable, or confirmed status.
• Population and setting: The geographic area, healthcare facility, workplace, or other population under investigation.
• Exclusion criteria: Circumstances that prevent an individual from meeting the case definition.

In the Crab Apple Valley scenario, suspected cases included people with relevant exposure on or after August 1 at the farmers’ markets or Community Hospital who subsequently developed symptoms such as fever, chills, weakness, cough, or painful and swollen lymph nodes.

A standardized case definition allows investigators to identify potential cases systematically, compare individuals using the same criteria, and monitor changes in the outbreak over time.

Ten Steps of an Epidemiologic Outbreak Investigation

An outbreak investigation generally follows a structured sequence, although several activities may occur simultaneously. Public health officials may begin control measures while continuing to gather evidence about the source and transmission of the disease.

1. Establish an Outbreak Investigation Team

The first step is to establish a multidisciplinary team capable of investigating the disease and implementing appropriate control measures.

In the Crab Apple Valley scenario, the team included healthcare professionals, epidemiologists, laboratory personnel, pharmacists, nurses, public health representatives, administrators, a statistician, and a public information officer.

Each discipline contributes different expertise. Epidemiologists analyze disease patterns and exposures, laboratory professionals support diagnostic testing, nurses contribute to surveillance and patient care, clinicians evaluate affected individuals, and communication professionals help deliver accurate public health information.

2. Establish That an Outbreak Exists

Investigators must determine whether the observed number of illnesses is greater than what would normally be expected for a particular population, location, and period.

Multiple individuals developing similar symptoms in Crab Apple Valley represented an unusual pattern requiring investigation. Establishing that an outbreak exists allows investigators to determine its magnitude, identify potential sources, and begin appropriate control activities.

3. Verify the Diagnosis

Before linking illnesses to a specific pathogen, investigators need to verify the diagnosis. Many infectious and noninfectious conditions can produce similar symptoms, making clinical and laboratory evidence important.

In the scenario, diagnostic testing supported infection with Yersinia pestis. Chest imaging also indicated pulmonary involvement among affected patients, supporting the possibility of pneumonic plague.

Verifying the diagnosis prevents investigators from basing epidemiologic analyses and public health interventions on an incorrect disease identification.

4. Develop a Case Definition

After the diagnosis has been evaluated, investigators establish criteria for suspected, probable, and confirmed cases.

The definition may incorporate symptoms, laboratory findings, geographic location, exposure history, and the relevant period. Consistent application of the definition improves case identification and ensures that epidemiologic calculations are based on comparable information.

5. Identify and Find Cases

Case finding involves locating people who meet or may meet the case definition. Investigators may review hospital records, laboratory reports, emergency department documentation, urgent care records, medical charts, and exposure histories.

Potential cases can also be identified through healthcare providers, emergency medical services, public health agencies, contact tracing, laboratory notifications, and interviews with potentially exposed individuals.

Rapid case finding is especially important during outbreaks involving diseases that can cause severe illness or spread through close contact.

6. Describe the Outbreak by Person, Place, and Time

Descriptive epidemiology focuses on three fundamental questions: Who is becoming ill? Where are cases or exposures occurring? When are illnesses developing?

Investigators may use line lists, interviews, medical records, geographic information, exposure histories, and epidemic curves to identify patterns.

For example, cases concentrated around a particular location may suggest a common exposure. Cases appearing in successive groups may indicate person-to-person transmission.

Person-place-time analysis therefore helps investigators develop hypotheses that can later be evaluated using analytic epidemiologic methods.

7. Develop and Evaluate Epidemiologic Hypotheses

Once investigators recognize patterns, they develop hypotheses about the source of infection and the mechanisms responsible for transmission.

In Crab Apple Valley, possible exposures included attendance at farmers’ markets and later contact with infected individuals in the hospital. Investigators could compare illness among people with different exposure histories to determine whether specific exposures were associated with disease.

This process helps distinguish plausible explanations from less likely hypotheses.

8. Implement Control Measures

Control measures should begin as soon as investigators have enough evidence to reduce the risk of additional transmission. Waiting for every question to be answered can allow an infectious disease to spread further.

Depending on the transmission pathway, measures in the scenario could include patient isolation, respiratory precautions, appropriate antimicrobial therapy, post-exposure prophylaxis for eligible contacts, and vector or environmental control.

9. Communicate Findings

Clear communication is essential during an outbreak. Public health officials should provide relevant information to healthcare professionals, patients, affected communities, public health agencies, and other stakeholders.

Communication should explain what is known, what remains uncertain, how people may have been exposed, which symptoms require attention, and what actions can reduce risk.

Information should be updated as investigators obtain additional evidence.

10. Maintain Ongoing Surveillance

Surveillance should continue after control measures are introduced. Monitoring new cases helps public health officials determine whether transmission is decreasing and whether additional interventions are needed.

Continued surveillance may also identify previously unrecognized cases and provide an early indication of renewed transmission.

Active and Passive Surveillance in an Outbreak

Surveillance is essential for detecting cases, monitoring disease trends, identifying changes in transmission, and evaluating public health interventions.

Active Surveillance

Active surveillance occurs when public health personnel actively search for cases instead of relying exclusively on routine reporting.

In the Crab Apple Valley investigation, active surveillance could include reviewing hospital and laboratory records, contacting healthcare facilities, interviewing exposed individuals, conducting contact investigations, and monitoring healthcare workers after potential exposure.

Active surveillance can improve the speed and completeness of case detection. However, it generally requires more personnel, time, coordination, and resources than passive surveillance.

Passive Surveillance

Passive surveillance depends primarily on routine reports from healthcare providers, hospitals, laboratories, and other reporting organizations.

For example, healthcare facilities may report suspected or confirmed plague cases to public health authorities. These routine reports can help officials identify unusual increases in disease.

Passive surveillance generally requires fewer resources, but delayed or incomplete reporting may result in missed cases.

Suspected Cases in the Crab Apple Valley Scenario

A suspected case is an individual who meets specified clinical and epidemiologic criteria but does not yet have enough evidence to be classified as probable or confirmed.

The scenario reports that 73 individuals presented to urgent care facilities with flu-like symptoms following potential exposure. Symptoms included fever above 38°C, chills, weakness, and tender or swollen lymph nodes.

Emergency medical personnel and hospital employees who had contact with the initial patient during resuscitation or clinical care were also considered potentially exposed.

Identifying suspected cases early allows investigators to initiate diagnostic evaluation, monitoring, contact investigation, and appropriate infection-prevention measures.

Probable and Confirmed Plague Cases

Case classification depends on the criteria established for a particular outbreak. A probable case generally meets specified clinical and epidemiologic criteria and may have supporting laboratory evidence. A confirmed case meets definitive laboratory or other criteria specified by the applicable case definition.

In the Crab Apple Valley scenario, one patient had a temperature of 103.2°F, chills, weakness, tender and swollen lymph nodes, and a positive rapid diagnostic test. Under the scenario’s classification system, these findings supported classification as a probable case.

Laboratory confirmation is important because fever, chills, weakness, cough, and lymph node swelling can occur with multiple diseases. Diagnostic testing helps establish whether illness is associated with Yersinia pestis.

How Plague Is Transmitted

Understanding transmission is a central component of outbreak investigation. Transmission describes how an infectious agent moves from a source or reservoir to a susceptible host.

Direct transmission occurs when an infectious agent passes directly from an infected person, animal, or source to another susceptible person. Depending on the pathogen, this can involve direct physical contact or exposure to infectious respiratory secretions.

Indirect transmission occurs when an intermediate mechanism is involved. Examples include vector, vehicle, and fomite transmission.

• Vector transmission: A living organism, such as a flea, mosquito, or tick, transmits the pathogen.
• Vehicle transmission: A contaminated substance or material, such as food, water, or equipment, contributes to exposure.
• Fomite transmission: A contaminated inanimate object contributes to transmission.

Transmission Pathways in Crab Apple Valley

The scenario suggests that initial infections were associated with vector-borne transmission. Jack Kalani’s work in an orchard environment with rodent activity created an opportunity for exposure to infected fleas carrying Yersinia pestis.

The later development of pneumonic plague introduced a potential respiratory transmission pathway. Pneumonic plague can spread through respiratory droplets during close contact with an infected person. This makes healthcare workers and other close contacts important populations for exposure assessment.

The scenario illustrates how different transmission pathways can contribute to different stages of a communicable disease outbreak.

Cumulative Incidence in Epidemiology

Cumulative incidence measures the proportion of an initially disease-free population at risk that develops a disease during a specified period.

The basic formula is:

Cumulative incidence = Number of new cases during the period ÷ Number of people at risk at the beginning of the period

The result can be multiplied by 1,000, 10,000, or 100,000 to make the measure easier to interpret.

Cumulative incidence is particularly useful when investigators want to estimate the risk of developing disease within a defined population over a specified period. During an outbreak, it can also help compare disease occurrence across different populations or exposure groups.

Cumulative Incidence Calculation for Crab Apple Valley

The scenario reports 93 new cases between August 1 and August 7 in a population of approximately 125,000 people.

Using a multiplier of 1,000:

Cumulative incidence = (93 ÷ 125,000) × 1,000

Cumulative incidence = 0.744 cases per 1,000 people

Rounded to two decimal places, the cumulative incidence is approximately 0.74 cases per 1,000 people during the specified period.

This means approximately 0.74 new cases occurred for every 1,000 people in the population during that period. Because incidence calculations depend on the population considered to be at risk, investigators should interpret the result in the context of the outbreak definition and available population data.

Purpose of the Crab Apple Valley Outbreak Investigation

The main purpose of the investigation is to identify the likely source of the outbreak, determine how Yersinia pestis was transmitted, measure the extent of illness, identify populations at risk, and implement interventions that can prevent additional infections.

Investigators need to determine who became ill, where and when exposures occurred, whether person-to-person transmission occurred, and which interventions can interrupt the identified transmission pathways.

The investigation can also support future prevention by informing environmental monitoring, vector control, infection-prevention practices, surveillance systems, community education, and emergency preparedness.

Observational Study Design for the Outbreak

The choice of epidemiologic study design depends on the research question, available data, population structure, and circumstances of the outbreak.

An experimental study would not be appropriate for intentionally determining whether an exposure causes plague because deliberately exposing people to a potentially severe infectious disease would be unethical. Observational study designs allow investigators to examine naturally occurring exposures and disease outcomes.

Retrospective Cohort Study

A retrospective cohort study can be useful when investigators can identify a defined population and determine whether individuals experienced a particular exposure before comparing disease occurrence among exposed and unexposed groups.

For example, investigators could review people who attended the farmers’ markets or had contact with infected individuals and determine whether specific exposures were associated with subsequent illness.

A retrospective cohort design can allow investigators to calculate attack rates and measures such as relative risk, helping evaluate associations between exposures and disease.

Epidemic Curve and Histogram

An epidemic curve, or epi curve, is a graph that displays cases according to the date or time when symptoms began. It is a key outbreak investigation tool because it provides information about the timing and possible pattern of transmission.

In the Crab Apple Valley scenario, the largest number of cases occurred around August 3. The early concentration of cases among people associated with the farmers’ markets may suggest a common exposure. Later cases associated with the hospital could indicate subsequent person-to-person transmission.

An epidemic curve can help investigators evaluate:

• The likely timing of exposure.
• The possible incubation period.
• Whether cases fit a common-source pattern.
• Whether person-to-person transmission may have occurred.
• Whether successive generations of transmission are present.

The curve therefore provides an important visual representation of how the outbreak may have developed over time.

Incubation Period and Its Role in Outbreak Investigation

The incubation period is the time between exposure to an infectious pathogen and the development of symptoms. Investigators use this interval to determine whether the timing of illness is consistent with a suspected exposure.

The incubation period varies by pathogen and clinical form of disease. Bubonic plague generally develops several days after exposure, while pneumonic plague may have a shorter incubation period.

In the Crab Apple Valley scenario, Jack Kalani’s initial illness is consistent with possible flea-associated exposure. His subsequent pulmonary disease created a potential opportunity for respiratory transmission among people who had close contact with him.

Understanding incubation periods helps investigators estimate exposure windows, interpret epidemic curves, identify contacts who may require monitoring, and evaluate competing explanations for an outbreak.

Endemic, Epidemic, Outbreak, and Pandemic Disease Patterns

Public health professionals use specific terminology to describe the frequency and geographic distribution of disease.

Endemic describes the usual or expected presence of a disease within a particular population or geographic area.

Epidemic describes disease occurrence that exceeds the expected level within a defined population, geographic area, or time period.

Outbreak generally refers to a localized occurrence of disease. Depending on the public health setting, outbreak and epidemic can sometimes be used similarly.

Pandemic describes an epidemic that spreads across multiple countries or continents and affects large populations.

The Crab Apple Valley scenario represents a localized outbreak because the increase in plague cases occurs within a defined community and period. It does not represent a pandemic because the scenario does not describe widespread international transmission.

Case Fatality Rate in the Crab Apple Valley Outbreak

The case fatality rate (CFR) is the proportion of identified cases that result in death from the disease during a specified period.

The formula is:

CFR = (Number of deaths among cases ÷ Number of cases) × 100

The scenario reports one death among 93 cases:

CFR = (1 ÷ 93) × 100

CFR = 1.08%

Therefore, the case fatality rate in the scenario is approximately 1.08%.

CFR should be interpreted cautiously because it can be influenced by case detection, diagnostic practices, treatment availability, disease severity, and the timing of the analysis. It should not automatically be interpreted as the individual mortality risk for every person who develops plague.

Primary Attack Rate

An attack rate is a form of cumulative incidence commonly used during outbreak investigations to describe disease occurrence within a defined population over a specific period.

The primary attack rate describes illness associated with an initial exposure or exposure event.

The scenario reports 62 cases among 200 people exposed at the farmers’ markets.

Primary attack rate = (62 ÷ 200) × 1,000

Primary attack rate = 310 cases per 1,000 exposed people

Expressed as a percentage:

(62 ÷ 200) × 100 = 31%

Therefore, the primary attack rate was 31%, or 310 cases per 1,000 exposed individuals.

This calculation indicates the proportion of the defined exposed group who developed illness during the specified outbreak period.

Secondary Attack Rate

The secondary attack rate describes disease occurrence among susceptible individuals exposed to a primary case or cases within a defined contact group and period.

The scenario reports 31 cases among 480 exposed hospital employees.

Secondary attack rate = (31 ÷ 480) × 1,000

Secondary attack rate = 64.58 cases per 1,000 exposed people

Expressed as a percentage:

(31 ÷ 480) × 100 = 6.46%

Therefore, the secondary attack rate was approximately 6.46%, or 64.58 cases per 1,000 exposed employees.

Comparing primary and secondary attack rates allows investigators to describe disease occurrence associated with the suspected initial exposure separately from illness occurring after contact with infected individuals.

Primary and Secondary Transmission in the Outbreak

In the scenario, primary infections were associated with exposure at the Crab Apple Valley farmers’ markets and environmental conditions surrounding the suspected initial source. Flea-borne transmission involving infected rodents was identified as the suspected mechanism for these initial infections.

Secondary transmission was associated with the later development of pneumonic plague. Healthcare workers who had close contact with infected patients, particularly those involved in resuscitation or direct patient care, could have been exposed to infectious respiratory secretions.

Distinguishing between primary and secondary transmission is important because different transmission pathways require different control strategies. Vector and environmental interventions address potential flea and rodent exposure, whereas isolation, respiratory precautions, contact monitoring, and appropriate prophylaxis can reduce the risk associated with subsequent person-to-person transmission.

Outbreak Control Measures

Effective outbreak control should address the suspected or confirmed transmission pathways and follow current public health and clinical recommendations.

Isolation and Respiratory Precautions

Patients with suspected or confirmed pneumonic plague should receive appropriate isolation and infection-prevention measures. Healthcare facilities should follow current public health and institutional guidance regarding respiratory precautions and personal protective equipment.

Reducing exposure to infectious respiratory secretions is particularly important when pneumonic disease and person-to-person transmission are suspected.

Antibiotic Treatment and Post-Exposure Prophylaxis

Prompt clinical evaluation and appropriate antimicrobial treatment are important components of plague management. Antibiotic selection should follow current clinical and public health recommendations and take the patient’s clinical condition into account.

People who experience relevant close exposure may require post-exposure prophylaxis depending on the type and duration of exposure and current public health guidance.

Vector and Environmental Control

Because the scenario identifies potential flea and rodent exposure, environmental assessment is an important component of outbreak control.

Public health authorities may evaluate rodent activity and flea populations and implement measures to reduce human exposure. Vector-control activities should be carefully coordinated because disturbing infected rodents can potentially increase exposure to infected fleas.

Communication Plan for the Plague Outbreak

Risk communication is an essential part of outbreak response. Healthcare professionals, public health agencies, patients, and community members need timely information that is accurate, understandable, and consistent.

Public health messages should describe relevant symptoms, potential exposure risks, preventive measures, and circumstances in which medical evaluation should be sought. Healthcare organizations should also receive current guidance related to diagnosis, reporting, infection prevention, and patient management.

An effective communication strategy may include public health alerts, healthcare-provider notifications, community education, coordination among public health agencies, and regular updates as the investigation develops.

Communication should distinguish confirmed findings from preliminary information. If recommendations change because new evidence becomes available, public health officials should explain the change clearly.

Nursing Implications for Communicable Disease Outbreaks

The Crab Apple Valley scenario illustrates how epidemiologic principles support the detection, investigation, and control of communicable diseases. Measures such as case definitions, surveillance, epidemic curves, incubation periods, cumulative incidence, attack rates, and case fatality rates provide different perspectives on an outbreak.

Nurses frequently play an important role in recognizing unusual symptoms, identifying clusters of illness, documenting exposure histories, and reporting suspected cases. Early recognition can help initiate public health investigation and infection-control measures.

Nurses also work closely with physicians, epidemiologists, laboratory professionals, infection-prevention specialists, pharmacists, administrators, and public health officials. This interdisciplinary approach is essential when responding to communicable disease outbreaks.

Nursing responsibilities during an outbreak may include:

• Recognizing and reporting suspected cases.
• Following isolation and infection-prevention procedures.
• Participating in surveillance and contact investigations.
• Collecting and communicating relevant patient information.
• Educating patients and families about prevention.
• Using personal protective equipment appropriately.
• Monitoring patients for changes in clinical condition.
• Supporting public health preparedness and response activities.

A strong understanding of epidemiology allows nurses to connect individual patient care with population-level prevention and evidence-based public health practice.

Key Takeaways From the Crab Apple Valley Outbreak

The Crab Apple Valley scenario demonstrates how several epidemiologic concepts work together during an infectious disease outbreak. A case definition establishes consistent criteria for identifying cases, while surveillance systems help public health professionals detect and monitor illness.

Person-place-time analysis helps investigators recognize patterns, and an epidemic curve provides a visual representation of when cases developed. Incubation-period information helps establish possible exposure windows, while cumulative incidence and attack rates measure disease occurrence in defined populations.

The case fatality rate provides information about deaths among identified cases, while transmission analysis helps investigators determine whether vector-borne, respiratory, or other pathways may be involved.

Successful outbreak response may require a combination of:

• Case identification and laboratory confirmation.
• Active and passive surveillance.
• Isolation and infection-prevention measures.
• Appropriate antimicrobial treatment.
• Post-exposure prophylaxis when indicated.
• Vector and environmental control.
• Contact investigation and monitoring.
• Clear and timely risk communication.
• Continued surveillance after interventions are implemented.

For nurses, epidemiologic knowledge supports early recognition, accurate reporting, patient education, infection prevention, interdisciplinary collaboration, and population-level disease prevention.

Frequently Asked Questions About Epidemiologic Outbreak Investigations

What is the purpose of an epidemiologic outbreak investigation?

An epidemiologic outbreak investigation aims to identify cases, determine the likely source and transmission pathway, measure the extent of disease, identify people at risk, implement control measures, and prevent additional cases.

What are the three components of descriptive epidemiology?

The three components are person, place, and time. They help investigators determine who is becoming ill, where cases or exposures are occurring, and when illness develops.

What is a case definition?

A case definition is a standardized set of clinical, laboratory, and epidemiologic criteria used to determine whether an individual should be classified as a suspected, probable, or confirmed case.

What is the difference between active and passive surveillance?

Active surveillance involves public health personnel actively searching for cases. Passive surveillance primarily relies on routine reports submitted by healthcare providers, laboratories, hospitals, and other reporting organizations.

What is an epidemic curve?

An epidemic curve is a graph showing cases according to the time or date of symptom onset. It can help investigators evaluate the timing of exposure and determine whether the outbreak pattern is consistent with common-source or person-to-person transmission.

How is cumulative incidence calculated?

Cumulative incidence is calculated by dividing the number of new cases during a specified period by the number of people at risk at the beginning of that period.

Cumulative incidence = New cases ÷ Population at risk

The result may be multiplied by a standard population factor, such as 1,000 or 100,000.

What is the case fatality rate?

The case fatality rate is the proportion of identified cases that result in death during a specified period.

CFR = (Deaths among cases ÷ Total cases) × 100

What is a primary attack rate?

A primary attack rate describes the proportion of people who become ill after an initial exposure or exposure event.

What is a secondary attack rate?

A secondary attack rate describes disease occurrence among susceptible individuals exposed to a primary case or cases within a defined contact group and period.

Why is epidemiology important in nursing?

Epidemiology helps nurses recognize disease patterns, report suspected cases, participate in surveillance and contact investigations, apply infection-prevention measures, educate patients, and contribute to population-level disease prevention.

References

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Centers for Disease Control and Prevention. (2024). About plague. https://www.cdc.gov/plague/about/index.html

Centers for Disease Control and Prevention. (2024). Plague. https://www.cdc.gov/plague/

Centers for Disease Control and Prevention. (2024). Surveillance resource center. https://www.cdc.gov/surveillance/

Munnangi, S., & Boktor, S. W. (2023). Epidemiology of study design. In StatPearls. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK470342/

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World Health Organization. (2024). Plague. https://www.who.int/news-room/fact-sheets/detail/plague