Each winter, strains of influenza A virus infect North Americans, causing an average of 36,000 deaths. Now, researchers say the virus comes from a viral reservoir somewhere in the tropics, settling a key debate on the source of each season's infection.

Outbreaks of the most common type of influenza virus, A (H3N2), are seeded by viruses that originate in East and Southeast Asia and migrate around the world, new research has found. This discovery may help to further improve flu vaccines and make the evolution of the virus more predictable.

The exchange of genetic material between two closely related strains of the influenza A virus may have caused the 1947 and 1951 human flu epidemics, according to biologists. The findings could help explain why some strains cause major pandemics and others lead to seasonal epidemics.

Low temperatures and relative humidities have been linked to the rapid spread of influenza in a new study by researchers, led by Dr. Peter Palese, from the Mount Sinai School of Medicine. The study, published in PLoS Pathogens, supports the theory of the seasonal flu.

Each year, seasonal flu kills approximately 36,000 people in the United States, most over age 70.Yet little is known about the benefit of influenza vaccine in older seniors. Randomized, placebo-controlled clinical trials involving seniors have not been large enough to reveal how well the vaccine can prevent death in this age group.

Regular as clockwork, the flu arrives every year. And, according to the national Centers for Disease Control and Prevention, 5 to 20 percent of the U.S. population on average will come down with it. About 36,000 people will die.