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Risk Factors and Causes


Dr. Flemings
Influenza, or the flu, is caused by an influenza virus. There are two types of influenza viruses: type A and type B. Captain Miller, can you tell us more about these viruses and how they spread?

Captain Miller
Absolutely, Dr. Flemings. Flu viruses spread mainly by tiny air droplets that are released when a person sneezes, coughs, or talks. These droplets can land in the mouths or noses of people who are nearby. Flu viruses can also spread through direct contact with a surface that has been exposed to the virus, such as a doorknob, hand railing, or toy.

It's possible to pass the flu on to someone else before symptoms develop, but people with the flu are generally most contagious in the first three to four days after their illness begins. Some people, especially young children and people with weakened immune systems, can remain contagious for seven days or longer after becoming sick.

Normally, nasal hairs and wet mucus trap foreign particles and microbes to protect the respiratory tract. In addition, the adenoids and tonsils, which are located in the pharynx, or throat, release white blood cells and antibodies that fight and destroy invading viruses.

However, influenza viruses are constantly changing, making it more difficult for the body's immune system to recognize and respond to them. Most often, these genetic changes in the viruses are gradual and accumulate over time. This is called antigenic drift. Occasionally, there is an abrupt, major change, called antigenic shift. When antigenic shift happens, most people have little or no protection against the new virus.