INFLUENZA

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae (the influenza viruses), that affects birds and mammals. The name influenza comes from the Italian influenza, meaning "influence" (Latin: influentia). The most common symptoms of the disease are chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.Fever and coughs are the most frequent symptoms.

In more serious cases, influenza causes pneumonia, which can be fatal, particularly for the young and the elderly. Although it is often confused with the common cold and other influenza-like illnesses, influenza is a much more severe disease and is caused by a different type of virus.Influenza may produce nausea and vomiting, particularly in children, but these symptoms are more common in the unrelated disease gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu".

People who contract influenza are most infective between the second and third days after infection and infectivity lasts for around ten days.Children are much more infectious than adults and shed virus from just before they develop symptoms until two weeks after infection. The transmission of influenza can be modeled mathematically, which helps predict how the virus will spread in a population.

Influenza can be spread in three main ways: by direct transmission when an infected person sneezes mucus into the eyes, nose or mouth of another person; through people inhaling the aerosols produced by infected people coughing, sneezing and spitting; and through hand-to-mouth transmission from either contaminated surfaces or direct personal contact, such as a hand-shake.The relative importance of these three modes of transmission is unclear, and they may all contribute to the spread of the virus. In the airborne route, the droplets that are small enough for people to inhale are 0.5 to 5 µm in diameter and inhaling just one droplet might be enough to cause an infection.Although a single sneeze releases up to 40,000 droplets,most of these droplets are quite large and will quickly settle out of the air.How long influenza survives in airborne droplets seems to be influenced by the levels of humidity and UV radiation: with low humidity and a lack of sunlight in winter probably aiding its survival.

As the influenza virus can persist outside of the body, it can also be transmitted by contaminated surfaces such as banknotes,doorknobs, light switches and other household items. The length of time the virus will persist on a surface varies, with the virus surviving for one to two days on hard, non-porous surfaces such as plastic or metal, for about fifteen minutes from dry paper tissues, and only five minutes on skin. However, if the virus is present in mucus, this can protect it for longer periods. Avian influenza viruses can survive indefinitely when frozen.