WebMD Medical Reference
Influenza — commonly shortened to “the flu” — is an extremely contagious viral disease that appears most frequently in winter and early spring. The infection spreads through the upper respiratory tract and sometimes invades the lungs.
The virus typically sweeps through large groups of people who share indoor space, such as schools, offices, and nursing homes. The global influenza epidemic of 1918 — which started in a military training camp in Kansas — eventually killed about 500,000 people in the United States and more than 20 million worldwide.
Although both colds and influenza stem from viruses that infect the upper respiratory tract, the symptoms of influenza are more pronounced and its complications more severe.
Influenza occurs most commonly in school-age children, but it’s most severe effects are felt by infants, the elderly and people with chronic ailments. Despite advances in prevention and treatment, influenza and its complications are still fatal to about 36,000 people in the U.S. annually. Specific strains of the disease can be prevented by a flu vaccine, or taking one of several types of medications. Some of these medications may also reduce the severity and the duration of the illness if started within the first 48 hours of symptoms.
A Cold or the Flu? Which Is It?
The common cold and influenza are both contagious viral infections of the respiratory tract. Although the symptoms can be similar, influenza is worse. A cold may drag you down a bit, but influenza can make you shudder at the very thought of getting out of bed.
Congestion, sore throat and sneezing are common with colds, and both ailments bring coughing, headache, and chest discomfort.
With influenza, you are likely to run a high fever for several days, and your head and body will ache. Usually, complications from colds are relatively minor, but a severe case of influenza can lead to a life-threatening illness such as pneumonia.
More than 100 types of cold viruses are known, and new strains of influenza evolve every few years. Since both diseases are viral, neither can be conquered by antibiotics, which only treat bacterial infections, but a few antiviral medications recently came available to help treat influenza. There are still no medications that specifically defeat the common cold – that one still has to run its course. Antibiotics may still be helpful but only to treat a secondary bacterial infection.
What Causes the Flu?
The flu virus is transmitted by inhaling droplets in the air that contain the virus, direct contact with respiratory secretions like sharing drinks or utensils, or by handling items contaminated by an infected person. In the latter case the virus on your skin then gets in mainly when you touch or rub your eyes, nose or mouth. Hand washing is an important way to limit the spread of influenza. The symptoms start to develop from one to four days after infection with the virus.
Researchers divide influenza viruses into three general categories: types A, B and C. Though all three types can mutate, or change into new strains, type A influenza mutates often, yielding new strains of the virus every few years. This means that you can never develop a permanent immunity to influenza. Even if you develop antibodies against a flu virus one year, those antibodies are unlikely to protect you against a new strain of the virus the next year.
Type A mutations are responsible for major epidemics every several years. Type B is less common and generally results in milder illness. However, major epidemics can occur with type B every three to five years.
Type C causes infection but does not cause typical flu symptoms. Both influenza A and B have been linked to the development of Reye’s syndrome, a potentially fatal complication that usually affects people under 18. Widespread outbreaks of Reye’s syndrome have occurred with influenza type B and also with chickenpox, but other viruses have been implicated. The risk is increased when taking aspirin, so children should not take aspirin during a viral illness.
Most influenza viruses that infect humans seem to originate in parts of Asia, where close contact between livestock and people creates a hospitable environment for mutation and transmission of viruses. Swine, or pigs, can catch both avian (meaning from birds or poultry) and human forms of a virus and act as hosts for these different viral strains to meet and mutate into new forms. The swine then infect people with the new form of the virus in the same way in which people infect each other – by transmitting viruses through exchange of droplets in the air.
Medically reviewed by Tracy Shuman, MD, July 2005.