When a disease outbreak occurs, responding swiftly and effectively is critical to protect public health and minimize the spread of disease. The following steps outline a systematic approach to investigating an outbreak, from confirming its existence to communicating findings for future preparedness. Each step is designed to ensure that the outbreak is thoroughly understood, controlled, and appropriately managed in coordination with national authorities, local communities, and other relevant stakeholders. By following these steps, public health responders can make informed decisions and take action to contain the outbreak and prevent further transmission.

Alignment with outbreak toolkit resources

Prepare and Respond

Conduct initial investigation to confirm the existence of an outbreak and verify the diagnosis

• Confirm that an unusual increase in cases is occurring beyond normal baseline by reviewing epidemiological data, engaging with healthcare workers, traditional practitioners, community health workers, and other relevant stakeholders with a One Health approach.
• Verify the diagnosis of the disease or condition causing the outbreak through clinical and laboratory confirmation.
  

Implement immediate and generic control measures, if possible

• Apply generic public health and social measures, to prevent further spread while the investigation is ongoing. Recommend generic prevention measures and engage the community in any decision-making processes throughout.
• Immediate control measures are implemented as soon as the suspected routes of transmission are identified.
  

Develop a case definition for the outbreak

• As soon as a suspected outbreak has been confirmed, an outbreak case definition must be agreed upon to enable the rapid identification of cases. 
• Define key attributes of the case definition including person, time, and place, clinical and epidemiological criteria, as well as laboratory diagnosis) to identify who is considered a case in the outbreak.
• When relevant classify and define the cases as a suspected case, probable case, or confirmed case.
  

Investigate further and adapt

Refine the case definition

• When more is known on the outbreak dynamic and context, the case definition may be refined.
• All changes to the case definition should be evidence-based, thoroughly communicated and logged to ease the interpretation of data and analytical findings.

Systemically find cases and contacts (when appropriate) and collect individual data using a standardized case investigation forms and a line-list

• Conduct active case finding through medical records, interviews, and fieldwork.
• Identify and trace contacts of confirmed cases.
• Collect data using standardized forms and maintain a line-list.
  

Conduct descriptive epidemiological analyses, describing time, place and person characteristics

• Create an epidemic curve (time distribution of cases).
• Map cases to identify geographic patterns.
• Analyse demographic characteristics (age, gender, occupation).

Develop hypotheses for the exposure, source and mode of transmission. Update the case definition, as appropriate

• Develop educated guesses on potential exposure, source, and mode of transmission based on the descriptive data.
• Refine the case definition based on new information.
• Consider the role of various risk factors.
  

Conduct further investigations to evaluate these hypotheses, including further laboratory investigations, environmental sampling and epidemiological studies to further define the agent, its mode of transmission, risk and the effectiveness of control measures

• Test the hypotheses through additional studies, lab tests, and environmental assessments to better understand the outbreak and refine control measures.
• Conduct analytical studies (e.g., case-control, cohort).
• Perform additional laboratory tests or environmental sampling.
• Assess the effectiveness of preliminary control measures.

Implement agent-specific control measures

• Targeted interventions (e.g., Vaccination, Quarantine, Sanitation).
• Monitor the implementation and impact of these measures.
• Adapt strategies as new evidence emerges.

Communicate

Develop public health messages with affected communities and the national public health authorities (or its equivalent), and other outbreak response actors

• Develop clear, accurate, and timely communication materials.
• Work with the community, health authorities, and other stakeholders to communicate risks, preventive actions, and updates on the outbreak.

Communicate the findings of the outbreak investigation more widely

• Share the results of the investigation with broader audiences, including scientific communities, public health organizations, and the public to inform future response efforts.