Journal of Pathogen Research

There are four sorts of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known because the flu season) almost every winter within the us . Influenza A viruses are the sole influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease. an epidemic can occur when a replacement and really different influenza an epidemic emerges that both infects people and has the power to spread efficiently between people. Influenza type C infections generally cause mild illness and aren't thought to cause human flu epidemics. Influenza D viruses primarily affect cattle and aren't known to infect or cause illness in people.

Influenza A viruses are divided into subtypes supported two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively). While there are potentially 198 different influenza A subtype combinations, only 131 subtypes are detected in nature. Current subtypes of influenza A viruses that routinely circulate in people include: A(H1N1) and A(H3N2). Influenza A subtypes are often further weakened into different genetic “clades” and “sub-clades.” See the “Influenza Viruses” graphic below for a visible depiction of those classifications.