Facts of Influenza

Overview

The influenza virus spreads mainly from person to person, and is generally considered to be contagious from one day before onset of symptoms to as much as a week after the infected person becomes sick. The virus is thought to spread mainly through the sneezes and coughs of infected people; rigorous hand washing and refraining from touching your own mouth or nose are both effective ways of reducing your risk of contracting the flu.

Types

There are three types of influenza viruses, classified as A, B and C. Type A influenza is divided into subtypes based on proteins, resulting in an H#N# subtype, where # is the subtype designation for the hemagglutinin (H) and neuraminidase (N) proteins. Influenza B viruses aren't subtyped but can be classified as different strains. Type C influenza usually results in a mild respiratory illness and, according to the Centers for Disease Control and Prevention, is not thought to cause epidemics. Types A and B, on the other hand, usually cause seasonal flu epidemics every winter in the United States and elsewhere.

H1N1

Both the "swine flu" which broke out in 2009 and the regular human influenza A virus are sub-typed as H1N1 viruses, but they are in fact very different. Usually strains of influenza that affect pigs do not affect humans, but in rare cases there may be transmission from pigs to humans and thus, sometimes, from human to humans. While the swine flu outbreak in 2009 has generated much media attention, it's not the only swine flu outbreak in history; there was a minor outbreak in 1976 of a swine flu virus, later subtyped as Hsw1N1, among soldiers at Fort Dix, New Jersey. Other influenza outbreaks have included the H5N1 "bird flu" virus.

Vaccine

Most seasonal flu vaccines are formulated to protect against the regular H1N1 and H3N2 strains of influenza A and the influenza B virus. The vaccine provides protection against viruses from the same or related strains as those in the vaccine. So, regular seasonal vaccines typically don't provide protection against influenza C, or against pandemic outbreaks, such as the 2009 H1N1 flu virus. Separate vaccines may be formulated for pandemic influenza strains, as is the case with the 2009 H1N1 virus.

Statistics

According to the CDC, between 5 percent and 20 percent of the U.S. population gets the flu in an average year. In that same average year, more than 200,000 people would be hospitalized for flu-related complications, and about 36,000 people would die from flu or flu-related causes. In years with novel influenza outbreaks, such as 2009's H1N1, more people may be sickened from influenza-related illnesses than in a usual year.

Symptoms

Influenza symptoms can include a high fever; headache; body aches; extreme fatigue; digestive symptoms, such as nausea, diarrhea and vomiting; and upper respiratory symptoms, including dry cough, sore throat and a runny or stuffy nose. Pneumonia, a possible complication of influenza infection that requires immediate medical care, may be characterized by a severe, phlegm-productive cough, high fever and sharp pain when taking a deep breath.