Influenza, often called "the flu," is caused by a type of virus that causes a respiratory illness that can range from mild to severe. Symptoms include fatigue, sore throat, stuffy nose, muscle aches and headaches according to the Centers for Disease Control and Prevention, or CDC. The CDC also notes that influenza is a contagious illness; infected persons are contagious for 1 day before feeling sick and up to 7 days after becoming ill. There are three main strains of the influenza virus: influenza A, influenza B and influenza C.
Influenza A
The Office of Occupational Safety at Louisiana State University, or LSU, notes that influenza A can infect horses, birds, pigs and humans. The types of two viral proteins determine the subtype of influenza A: hemagglutinin, H, and neuraminidase, N. There are 16 known H proteins and 9 known N proteins.
Flu.gov, a website sponsored by the U.S. Department of Health and Human Services, reports that a type of influenza A strain called H1N1, also known as "swine flu," is a particularly severe type of influenza A. In addition to normal flu-like symptoms, H1N1 can cause severe and/or persistent vomiting; inability to wake-up; trouble breathing; skin that is blue or gray; and difficulty drinking enough fluids. There are many other types of influenza A as well. LSU notes that current circulating strains of influenza A include H1N1, N1N2 and H3N2.
Influenza B
Unlike influenza A, influenza B is not named by the H and N designation. Influenza B usually causes a milder type of disease than influenza A according to the online medical library Merck Manuals. Furthermore, influenza B virus usually does not cause widespread epidemics. However, every 3 to 5 years, influenza B does cause epidemics of more moderate or severe illness. LSU adds that influenza B only infects only humans and not other animals.
Influenza C
LSU describes that influenza C, also not named using the H and N designation, causes a mild illness, and thus is not a public health threat. Furthermore, this strain of influenza also does not cause epidemics. The Nieman Foundation for Journalism at Harvard adds that influenza C can infect humans, dogs and pigs.
