- Influenza (the flu) is highly infectious and contagious. It causes considerable illness in the United States each year. The most severely affected are the very young, sick, and elderly. Researchers want to find healthy volunteers for influenza "challenge studies." In these studies, doctors expose a person to a flu virus. Then they study the flu through the body's natural healing process. This information will help to find better ways to prevent the flu. It may also improve the treatment of people who get the flu.