The flu is a blanket term given to respiratory infections that can come from a variety of different, but related, viruses. There are three major types of influenza viruses, with two of them responsible for the seasonal flu outbreaks that occur most years. Some forms of the influenza virus normally infect non-humans but can, on occasion, infect humans as well. Flu treatment hinges on both vaccination and symptomatic relief.
Identification
The flu is similar to the common cold in that it is a viral respiratory infection. However, the symptoms of the flu are more severe and can lead to patients developing a fever, a headache and extreme exhaustion. The Center for Disease Control and Prevention estimates that between five and 20 percent of all Americans get the flu each year.
Time Frame
The most common form of the flu is called "seasonal" flu. This type of flu typically affects most people during the winter and late fall. Seasonal epidemics spread rapidly through communities, with the number of cases peaking after three weeks. Seasonal flu epidemics typically subside three to four weeks after their peak. Families with school-age children typically have a greater risk of developing the flu because of the chance that the seasonal flu will sweep through a school.
Types
There are three main types of influenza viruses. Type A is the form that can cause deadly influenza epidemics that break out worldwide every 10 to 40 years. Type B outbreaks typically cause smaller, more localized outbreaks. Both types are able to cause seasonal influenza. Type C viruses are not associated with large-scale outbreaks.
H1N1 and Bird Flu
Some forms of influenza can cross species to infect humans. This is the case with both the H1N1 and "bird" (also known as H5N1) influenza viruses. These viruses only rarely affect humans and generally do not cause serious infections. However, there is the risk that a cross-species infection could result in the virus mutating to infect human more easily, leading to a pandemic.
Treatment
The front-line treatment for flu viruses is vaccinations. Each year flu shots are widely available to help prime the immune system against flu viruses. Other treatments include the medications oseltamivir and zanamivir, which can be used to shorten the duration of an infection from either a type A or type B influenza infection.
Antigenic Drift
The human immune system is adept at preventing viruses from infecting more than once via the production of "memory" immune cells. These white blood cells remain in the circulation for a long period of time (often many years) to allow the immune system to rapidly respond to additional outbreaks of a flu virus. However, the viruses that cause influenza can mutate, which can minimize the effectiveness of these immune cells. These mutations are also what make flu vaccines from previous years less effective, forcing scientists to develop new vaccines each year.
