Flu refers to illnesses caused by a number of different influenza viruses. Flu can cause a range of symptoms and effects, from mild to lethal.

Two strains of flu, seasonal flu and the H1N1 (Swine) flu, are currently circulating in the United States. A third, highly lethal H5N1 (Bird) flu is being closely tracked overseas.

Most healthy people recover from the flu without problems, but certain people are at high risk for serious complications.

Extensive efforts are underway to track and monitor the spread of all flu viruses. In the U.S., epidemiologists at the Centers for Disease Control (CDC) are working with states to collect, compile and analyze reports of flu outbreaks. More on the current situation.

Flu symptoms may include fever, coughing, sore throat, runny or stuffy nose, headaches, body aches, chills and fatigue. In H1N1 flu infection, vomiting and diarrhea may also occur.

Annual outbreaks of the seasonal flu usually occur during the late fall through early spring. Most people have natural immunity, and a seasonal flu vaccine is available. In a typical year, approximately 5 to 20 percent of the population gets the seasonal flu and approximately 36,000 flu-related deaths are reported.

This year, the H1N1 flu virus may cause a more dangerous flu season with a lot more people getting sick, being hospitalized and dying than during a regular flu season. H1N1 is a new virus first seen in the United States. It is contagious and spreads from person to person. Like seasonal flu, illness in people with H1N1 can vary from mild to severe.

A flu pandemic occurs when a new influenza A virus emerges for which there is little or no immunity in the human population; the virus causes serious illness and spreads easily from person-to-person worldwide. On June 11, 2009, the World Health Organization (WHO) declared that a global pandemic of H1N1 flu is underway.

H5N1 (Bird) flu is an influenza A virus subtype that is highly contagious among birds. Rare human infections with the H5N1 (Bird) flu virus have occurred. The majority of confirmed cases have occurred in Asia, Africa, the Pacific, Europe and the Near East. Currently, the United States has no confirmed human H5N1 (Bird) flu infections, but H5N1 (Bird) flu remains a serious concern with the potential to cause a deadly pandemic.