The flu is a respiratory (nose, throat, and lung) infection that can be caused by a variety of influenza viruses. Many people use the word "flu" when they actually have a cold. Although the common cold is also caused by viruses, the flu and common cold differ in several ways.
In North America, flu almost always strikes between November and April. Up to 25% of the population may be infected in an average year. Stronger epidemics (i.e., when the flu occurs in more people than expected in a given area or season) come every 2 or 3 years, infecting twice as many people as during an "off" year.
Most people who get the flu will recover within 1 to 2 weeks, but some people are at risk of developing complications such as pneumonia. In any one year in the US, from 3,000 to 49,000 people die from complications of influenza, and about 200,000 people are hospitalized with flu-related complications. Most of these people have other medical conditions, are seniors, or are very young children.
Influenza is contagious, which means it can be spread easily from person to person. Viruses that cause influenza spread from person to person mainly by airborne droplets of respiratory fluids that are sent through the air when someone infected with the virus coughs or sneezes. Other people inhale the airborne virus and can become infected.
In some cases, the flu can be spread when someone touches a surface (e.g., doorknobs, countertops, telephones) that has the virus on it and then touches his or her nose, mouth, or eyes. The flu is most easily spread in crowded places such as schools and offices.
There are three families of influenza virus: A, B, and C. Type C more commonly affects ducks, geese, turkeys, and chickens, but it has also been involved in a small percentage of human cases. Type B mainly affects humans and causes a milder disease, and it changes very little from year to year.
Type A influenza poses the most serious problems for humans. Strains of this type have also been found in birds, humans, horses, pigs, seals, whales, and ferrets. Viruses that affect two different species sometimes combine and mix-and-match genetic information to create a new strain that nobody is immune to and for which no vaccine has been prepared.
There are infinite new varieties of type A influenza. Avian flu is a type of influenza A that had been seen only in birds, including chickens and ducks, until 1997 when the first human case was reported in Hong Kong. Although the avian flu affected mostly animals, it did cause a few cases of severe disease with a high risk of death in humans. Many birds were slaughtered in Hong Kong to prevent the disease from spreading. In early 2004, outbreaks of the avian flu reemerged in Japan, South Korea, Vietnam, Thailand, Cambodia, China, and Indonesia. Human cases have been reported in Vietnam and Thailand. Millions of chickens have been affected and killed in the countries listed in an attempt to stop the spread of the disease.
Swine flu is another type of influenza A that is normally found in pigs. Swine flu is passed from pig to pig, and although it usually only infects pigs, there have been periodic infections in humans, resulting in H1N1 flu virus (human swine flu). Most of these cases occur in people with direct exposure to pigs (e.g., people working on pig farms). Person-to-person transmission of the H1N1 flu virus (human swine flu) does occur, but it is not clear how easily the virus is spread among people.
The flu takes 1 to 4 days to incubate in humans, but infected people become contagious before symptoms appear, often just the day after the virus enters the body. Adults remain infectious (i.e., they can spread the virus to others) for about 6 days, and children remain infectious for up to 10 days.