There are three types of influenza viruses that infect humans. These are designated A, B, and C. According to the Centers for Disease Control and Prevention, types A and B can produce serious illness and cause epidemics, while type C causes only mild respiratory infections and are not thought to cause epidemics.
The annual flu shots currently contain three different strains of influenza virus, two for type A and one for type B.
Influenza B viruses are the predominant circulating strain approximately one out of three years, according to Dr. David Jackson and colleagues in the UK. These get less publicity than the type A viruses that also infect birds and swine. The natural reservoir of animal infection gives influenza A the potential for pandemic infection, when new strains emerge from their animal hosts and jump to humans.
Type B has no known animal host, and less potential for pandemics.