Influenza is an acronym for influenza. The main reason for frequent influenza epidemics is that the influenza virus itself is a readily mutable virion that is capable of producing many new subtypes. Influenza is spread through the air mainly by droplets from coughing or sneezing, and is easily spread to healthy people around. Because influenza viruses change quickly and are prone to antigenic mutation, even after a person is exposed to or infected with the virus, he or she does not have the appropriate antibodies and resistance to the antigen of the virus, so he or she will repeat the next time a new influenza virus appears and become infected. Moreover, influenza viruses are universally susceptible to human beings and can easily lead to epidemics and pandemics.