The influenza virus has three classifications; type A, B and C. Influenza spreads rapidly and can occur as local or regional epidemics or global pandemics. While becoming infected with any strain of influenza virus leads to immunity to that strain, the A and B viruses can change their structures periodically. These alterations prevent your immune system from recognizing the virus. Antibodies formed to fight the previous strain of the virus will be ineffective against the new one. For this reason, you can become sick with influenza types A and B more than once during your lifetime.
Influenza A
This virus is associated with the most serious epidemics. The virus has altered its structure several times over. 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 the already existing immunity in populations can be insufficient to prevent epidemics. H1N1 is an influenza A virus.
Influenza B
Influenza B virus has generally milder symptom than those caused by influenza A, and, unlike influenza A, which is essentially a winter-spring virus, influenza B is a year-round virus. According to the Centers for Disease Control and Prevention, there are rapid diagnostic tests that can usually identify, and/or distinguish between, influenza A and influenza B viruses.
Influenza C
Type C viruses cannot change their structure, so once a type C virus has infected a person, they acquire long-lasting immunity to it. In contrast to influenza A and B, C usually causes mild respiratory symptoms that are indistinguishable from the common cold. Nevertheless it is still a contagious respiratory viral infection that is easily spread through airborne respiratory droplets.
