An outbreak of a disease can affect a handful of people, such as the healthy Navajo adults stricken with hantavirus in 1993, or thousands of people, such as those infected with influenza in 1918.
Microbes spread through local populations by different routes. Some spread through shared resources, such as air and water. Others spread through food handling, shared needle injections, sexual contact, or disease carriers, such as fleas and rats.
