Excerpted from Wikipedia
Influenza, commonly known as “the flu“, is an infectious disease caused by an influenza virus. Symptoms can be mild to severe. The most common symptoms include: high fever, runny nose, sore throat, muscle and joint pain, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks. In children, there may be diarrhea and vomiting, but these are not common in adults. Diarrhea and vomiting occur more commonly in gastroenteritis, which is an unrelated disease and sometimes inaccurately referred to as “stomach flu” or the “24-hour flu”. Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.
Three of the four types of influenza viruses affect humans: Type A, Type B, and Type C. Type D has not been known to infect humans, but is believed to have the potential to do so. Usually, the virus is spread through the air from coughs or sneezes. This is believed to occur mostly over relatively short distances. It can also be spread by touching surfaces contaminated by the virus and then touching the eyes, nose, or mouth. A person may be infectious to others both before and during the time they are showing symptoms. The infection may be confirmed by testing the throat, sputum, or nose for the virus. A number of rapid tests are available; however, people may still have the infection even if the results are negative. A type of polymerase chain reaction that detects the virus’s RNA is more accurate.
Frequent hand washing reduces the risk of viral spread, as does wearing a surgical mask. Yearly vaccinations against influenza are recommended by the World Health Organization (WHO) for those at high risk, and by the Centers for Disease Control and Prevention (CDC) for those six months of age and older. The vaccine is usually effective against three or four types of influenza. It is usually well tolerated. A vaccine made for one year may not be useful in the following year, since the virus evolves rapidly. Antiviral medications such as the neuraminidase inhibitor oseltamivir, among others, have been used to treat influenza. The benefit of antiviral medications in those who are otherwise healthy do not appear to be greater than their risks. No benefit has been found in those with other health problems.
Influenza spreads around the world in yearly outbreaks, resulting in about three to five million cases of severe illness and about 290,000 to 650,000 deaths. About 20% of unvaccinated children and 10% of unvaccinated adults are infected each year. In the northern and southern parts of the world, outbreaks occur mainly in the winter, while around the equator, outbreaks may occur at any time of the year. Death occurs mostly in high risk groups—the young, the old, and those with other health problems. Larger outbreaks known as pandemics are less frequent. In the 20th century, three influenza pandemics occurred: Spanish influenza in 1918 (17–100 million deaths), Asian influenza in 1957 (two million deaths), and Hong Kong influenza in 1968 (one million deaths). The World Health Organization declared an outbreak of a new type of influenza A/H1N1 to be a pandemic in June 2009. Influenza may also affect other animals, including pigs, horses, and birds.