There are three different types of influenza virus. Types A and B tend to spread rapidly and can occur either in regional outbreaks called epidemics, or worldwide outbreaks called pandemics. Type C influenza virus is the least severe of all three. Infection with any strain of influenza virus leads to immunity to that particular strain. However, types A and B can periodically change their structures, preventing the immune system from recognizing the virus. Antibodies developed from the previous strain of the virus will be ineffective against the new one. For this reason, an individual can become sick with a particular influenza, type A or B, more than once during a lifetime.
Influenza Type A
The type A virus, which has altered its structure many times, causes the most serious influenza-related epidemics. These changes occur in the surface antigens, the triggers of the immune response. This major alteration of the virus, called antigenic shift, has led to several pandemics. In addition, both influenza A and B viruses can undergo antigenic drift, in which relatively minor changes in their surface structures occur. These minor changes do not usually lead to pandemics, but can weaken the immunity that exists in populations, enough to cause epidemics. According to the Centers for Disease Control and Prevention, in 2010, the H1N1 influenza A virus caused an influenza pandemic.
Influenza Type B
Influenza B infections are generally milder than those caused by the type A virus, and similar to types A and C, there is no cure. Most doctors recommend only the use of acetaminophen to relieve the symptoms associated with the flu such as a sore throat, headache, or to reduce a fever. Children or adolescents with influenza or a fever should not be given aspirin because of its association with Reye's syndrome, a potentially life-threatening disorder. According to the University of Maryland Medical Center, up to 20 percent of America's population comes down with the flu annually.
Influenza Type C
Type C viruses cannot change their structure, so, once infected by a type C influenza, an individual has acquired a long-lasting immunity to it. According to the Centers for Disease Control and Prevention, the influenza type C virus usually causes a mild respiratory illness that is often indistinguishable from the common cold. Most influenza vaccines will not protect an individual against the influenza C virus. Common symptoms of all types of the influenza virus include fever, headache, fatigue and sometimes a sore throat, sneezing, cough, aches and pains and runny nose. The severity of these symptoms depends on the type or strain of influenza virus.
