Influenza
Cause Of The Disease Influenza is an infectious disease caused by the influenza virus. ... There are three types of influenza viruses: influenza A, B and C. Influenza A can infect humans and other animals while influenza B and C infect only humans. Influenza C virus causes a very mild illness and does not cause epidemics. The two types of influenza virus that are important for humans, types A and B, change almost continually. Two genetic mechanisms are involved: Small changes in the surface properties of the influenza virus occur frequently because of genetic mutations, but the new virus strain produced in this way may not be greatly different from a strain that circulated the previous year. ... The second mechanism, which occurs only with type A influenza viruses, involves the exchange of genes between two viruses: one being a human flu virus, the other usually a bird or pig flu virus.