Flu in Adults Causes

Three types of influenza viruses exist. Types A and B cause epidemics of severe respiratory illnesses known as "the flu," and type C causes a mild illness, not associated with epidemics. Type A is divided into different subtypes based on the chemical structure of the virus. The H1N1 swine flu virus is a type A influenza virus. Type B is not divided into subtypes. Both type A and type B are responsible for the seasonal outbreaks of flu.

• Outbreaks occur more frequently in the winter months. Many factors may play a role in this seasonal pattern:



• The virus survives for longer periods indoors in winter because the relative humidity of indoor air is very low, in comparison to the outside air.



• The virus may stay suspended in the air for prolonged periods and thus infect others by being inhaled. The virus droplets can also infect by landing on sensitive body areas such as the eyes, nose, or mouth.



• In winter, humans tend to be indoors more and thus have closer contact with each other, which makes it easier for the virus to spread.



• Flu outbreaks are classified as epidemics (occurring in a set geographical area) or pandemics (a worldwide occurrence). A flu pandemic can occur when a new influenza A virus emerges against which there is very little immunity already in the human population. Because there is little immunity, the new virus can spread from person to person very easily and can sicken more people. In 2009, a pandemic influenza strain began circulating called H1N1 or swine flu.



• Influenza is a highly contagious disease. The virus is spread when you either inhale infected droplets in the air (spread when an infected person coughs or sneezes) or when you come in direct contact with an infected person's secretions (for example, by kissing, sharing of handkerchiefs and other items, and through use of objects such as spoons and forks). Touching smooth surfaces, such as doorknobs, handles, and telephones, are other ways to transfer the virus to your hands, which may then contact the nose or mouth where the virus gets absorbed.



• A person with flu is contagious for up to seven days after the onset of the illness, although the virus can be detected in the secretions up to 24 hours before the onset of symptoms. Thus, an individual can transmit the virus one day before symptoms begin.



• In young children, the virus can still be spread in the secretions into the second week of illness.