A disease outbreak occurs when a pathogen (virus, bacteria, or parasite) infects many people. These infected individuals spread the disease by direct contact with each other or with contaminated objects, such as a cough or sneeze of an infected person. Overcrowded conditions, poor regional design and hygiene due to poverty, climate change and natural disasters, and poor water supply can lead to a high risk of disease transmission.
When a disease outbreak happens, it requires a global response to control its spread. CDC investigates and tracks outbreaks with partners across the globe, using a One Health approach to detect, respond, and manage diseases, infections, and toxins.
Detection of disease outbreaks begins with identifying an unexpected increase in cases compared to historical data. This is often done by comparing current case counts to incidence rates from health department surveillance and disease registries, but variations in reporting procedures and changes in the overall population size must be taken into account.
Once a suspected outbreak is identified, investigators verify the number of new cases by collecting clinical and laboratory evidence. They also monitor local and global factors that may influence the outbreak’s spread, such as travel patterns, weather conditions, demographic shifts, and underlying social dynamics. For example, during the Ebola epidemic in West Africa, political instability and heightened social tensions amplified the outbreak’s impact by making it difficult to enforce quarantine and limit contact within communities.