Although new variations of influenza, or the flu, arise every year, there are only three classifications of the flu, according to the Centers for Disease Control and Prevention: A, B or C viral infections. Avery form of the flu, including avian or bird flu, or H1N1 or swine flu, falls under these categories. Each category also includes subcategories of various strains.
Influenzavirus C
The C viral strain of influenza is the least severe form of the infection, usually only causing mild problems such as respiratory illnesses. The Centers for Disease Control and Prevention report that this type of flu strain does not generally cause wide-spread cases or epidemics of illness. For the most part, the C virus affects small populations such as students in a school or people in a town.
Influenzavirus B
More common than the C virus, the B virus can cause some wide-spread infections. People exposed to the B virus flu strain when young often develop an immunity to it. Despite that, variations of the B virus can cause epidemics that result in many people getting sick. The seasonal flu shots available to protect people from getting influenza usually combat this flu strain as well as the influenza C strain. Symptoms of the B virus include fevers, chills, rhinitis, congestion and a sore throat. The C strain produces the same symptoms to a lesser degree, while the symptoms typically occur more severely with the A strain.
Influenzavirus A
The most virulent of the flu strains, the A virus causes greater spread of influenza classified as a pandemic. The great influenza world pandemic that followed World War I, also called Spanish Flu, infected one-fifth of the global population, reports Stanford University. That included approximately 10 times the number of Americans as were killed during the war. The more recent H1N1, or swine flu, called “a distant cousin” of another H1N1 flu strain, the Spanish Flu, spread quickly around the world in 2009.
