Influenza is a virus that causes respiratory infection, but the symptoms affect the whole body. FluFacts.com, an informational website sponsored by the biotechnology company Genentech, describes these symptoms as headache, high fever, sneezing, aches and pains, fatigue, and a cough. Influenza causes widespread infection epidemics, and Ohio State Medical Center estimates that 5 to 20 percent of the U.S. population is infected each year. There are three main types of influenza viruses: influenza A, influenza B, and influenza C.
Influenza A
Influenza A is the most prevalent type of influenza and has caused the most serious epidemics in U.S. history, according to FluFacts.com. The Centers for Disease Control and Prevention, or CDC, note that influenza A is further separated into subtypes with names on based two different viral proteins, hemagglutinin, H, or neuraminidase, N. Scientists have identified16 different H proteins and 9 different N proteins. In addition to the subtypes, individual populations of the virus belong to specific strains of the subtype. For example, the 2009 H1N1 virus is one strain of the subtype H1N1. This strain of H1N1 made many people sick in the spring of 2009. Influenza A proteins that belong to the previous year’s strain are included in the seasonal flu vaccine to generate immunity to the flu virus.
Ohio State University Medical Center adds that Influenza A can cause such a serious illness that infected persons need to be hospitalized. Severe cases of influenza A infection can cause pneumonia, in which the lung tissue becomes inflamed, and death.
Influenza B
Like influenza A, influenza B can cause widespread epidemics, but with milder symptoms, according to FluFacts.com. Though influenza B is not categorized into subtypes by H and N protein type, there are different strains of influenza B. Because of its ability to spread quickly through the population, the proteins of the previous year's strain influenza B also are included in the seasonal vaccine.
Influenza C
This subtype of influenza causes a very mild respiratory illness and does not cause widespread epidemics, according to the CDC. The symptoms resemble the common cold, according to FluFacts.com. Because this virus poses little threat to public health, these proteins are not included in the seasonal flu vaccine.
