The flu is an infection caused by a virus in the upper breathing passages that may also spread to the lungs. Symptoms include fever, headache, general aches and pains, and extreme exhaustion. The flu is a contagious disease in which the influenza virus is spread through the air by the coughing and sneezing of people who are already infected. The virus is inhaled into the lungs where it attacks tissues involved in breathing. People with the flu usually feel sick rather suddenly, with symptoms that may include fever, headache, and feeling very tired. These symptoms are sometimes followed by coughing and a stuffy or runny nose.
Once the virus enters your body, it is usually one to four days before you start to feel symptoms. The infection lasts until the body’s immune system can build up a response to the virus and mount a counterattack, usually an additional four to five days.
Between 35 and 50 million Americans come down with the flu each flu season. Because the types (or strains) of the influenza virus that cause the flu may vary each season, the severity of symptoms and the number of people who become sick also differs. About 10% to 15% of adults in a typical community and a higher percentage of children will come down with the flu.
Outbreaks of flu occur almost every winter in the northern and southern hemispheres, and can show up year-round in the tropics. Doctors do not know where or how the virus lives between outbreaks. Epidemics usually begin suddenly, peak during a two- to three-week period, and last from two to three months. The first signs of a flu outbreak are usually children with a cough and temperature. Then, adults start to come down with the disease.
Every 10 to 15 years, people around the world will suffer from the flu. These pandemics, another name for worldwide epidemics, occur when completely new strains of the virus emerge. People become sick because they have not built up immunity to the new type of virus. The pandemic of 1918 killed 500,000 people in the U.S., and more than 20 million people worldwide. Half of the world's population became infected. The last true pandemic took place in 1968-1969. Considered the mildest pandemic of the 20th century, it did not produce an increase in the expected number of flu-related deaths in the U.S. Pandemics can last for several winters until enough people build immunity to the strain.
The virus that causes the flu spreads through lung and nasal secretions. When someone with the flu coughs or sneezes, some of the virus is expelled into the air, and another person can breathe it in. The flu can also be caught by someone touching an object that another person has sneezed on, and then bringing that hand to his or her face, especially the eyes. Influenza viruses can stay alive in the air for up to three hours. Being in close quarters (in an elevator for example) with someone who has the flu will increase the likelihood of catching the disease. Being in crowded places during the flu season also increases the risk of contagion. Usually someone with the flu stops “shedding” the virus after two to five days from the start of symptoms. Until then, other people can pick up the disease through contact with the patient’s secretions. Hand-to-hand and other personal contact may also spread the virus. Once passed along, the virus grows for one to four days before symptoms develop.
Three different types of flu viruses cause influenza: influenza A, B, and C. Generally, the worst flu symptoms and 90% of cases of influenza-related pneumonia are caused by influenza A. Influenza B generally produces milder symptoms. However, during an epidemic, even influenza B can cause severe sickness and possibly death. Since these types of flu viruses vary yearly, they are named after the place or animal in which they are thought to have originated—for example, the Asian flu, Hong Kong flu, and swine flu. Influenza C rarely causes human disease, and is more common in swine.
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