Important update: Healthcare facilities
CDC has updated select ways to operate healthcare systems effectively in response to COVID-19 vaccination. Learn more
To maximize protection from the Delta variant and prevent possibly spreading it to others, get vaccinated as soon as you can and wear a mask indoors in public if you are in an area of substantial or high transmission.
UPDATE
Given new evidence on the B.1.617.2 (Delta) variant, CDC has updated the guidance for fully vaccinated people. CDC recommends universal indoor masking for all teachers, staff, students, and visitors to K-12 schools, regardless of vaccination status. Children should return to full-time in-person learning in the fall with layered prevention strategies in place.

Identifying the source of the outbreak

Identifying the source of the outbreak
Want to learn more about epidemiology?
Check out the resources at CDC’s
Introduction to Epidemiology

The novel (new) coronavirus that first appeared in China had never been seen before, so it quickly gained the attention of scientists around the world.

Epidemiologists did field investigations to find out how the new virus started. They conducted surveys in the community and in health facilities and collected nose and throat specimens for lab analyses. These investigations showed them who was infected, when they became sick, and where they had been just before they got sick.

Using this information, epidemiologists determined that the virus possibly came from an animal sold at a market. The new virus was found to be a coronavirus, and coronaviruses cause a severe acute respiratory syndrome. This new coronavirus is similar to SARS-CoV, so it was named SARS-CoV-2 The disease caused by the virus was named COVID-19 (COronVIrusDisease-2019) to show that it was discovered in 2019.

An outbreak is called an epidemic when there is a sudden increase in cases. As COVID-19 began spreading in Wuhan, China, it became an epidemic. Because the disease then spread across several countries and affected a large number of people, it was classified as a pandemic.

Terms to know

Epidemic: Sudden increase in cases of a disease.

Epidemiology:  Study of disease and other health outcomes, their causes in a population, and how they can be controlled.

Outbreak: A higher-­than-expected number of occurrences of disease in a specific location and time.

Pandemic: Event in which a disease spreads across several countries and affects a large number of people.