We usually think of the flu as an illness that afflicts the elderly. But this season the virus seems to be hitting younger people hard.

This winter at Duke University Medical Center in Durham, N.C., the median age of people hospitalized with influenza was 28.5 years. Many of the worst cases of flu occurred in young, otherwise healthy people.

Fifteen years ago an unwelcome viral visitor entered the U.S., and we've been paying for it ever since.

The U.S recorded its first case of West Nile virus back in 1999. Since then, the disease has spread across the lower 48 states and cost the country around $800 million, scientists reported this week in the American Journal of Tropical Medicine and Hygiene.