True influenza is caused by one of the influenza viruses.
Influenza spreads easily from person to person, so it normally appears as an epidemic.
There are also isolated cases of the disease. Influenza epidemics , for reasons that are unknown, nearly always occur during cold weather. Cause There are three main types of influenza virus, categorized as types A, B and C. Type C is the mildest. Once an individual has gone through a course of infection and recovery, he or she is likely to have enough antibodies in the blood to resist further type C infections for life. These changes are sufficient to make the antibodies developed in response to previous infections irrelevant.