Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Natural history timeline. Modified with permission from Centers for Disease Control and Prevention. Principles of Epidemiology. 2nd ed. Atlanta, GA: US Department of Health and Human Services; 1992. Copyright 1992, Centers for Disease Control and Prevention.
