The enters for Disease Control breaks influenza strains into three categories: A, B and C. Within each of these, there may be variations and subtypes of flu that can mutate and infect people. The classifications separate the flu strains according to their severity and contagion levels. The mutation of flu viruses is what causes the biggest problems for the medical community because each mutation means a different variation of vaccination must be developed.
Strain A
Influenza A, the CDC states, is the most serious of the three types. The H1N1, or swine flu, is a variation of influenza A, as is bird flu, also known as H5N1. Swine flu appeared in early 2009 and caused the World Health Organization to declare the potential for a pandemic. A pandemic is the widest possible spread of a flu strain, meaning it infects people around the world. Swine flu first was identified in Mexico but then spread to most other countries. Avian or bird flu was a mutation of a disease that previously only affected birds. It began infecting people in 1997 in Asia. Both are deadly diseases.
More commonly, however, influenza A is typically the seasonal flu that affects many people but is not deadly. Most influenza A strains are believed to be descendants of the Spanish Flu pandemic that killed 40 to 50 million people between 1918 and 1919.
Strain B
Influenza B typically does not spread to as many people as the A strains. It also generally is not as unpleasant as the stronger strains and causes milder symptoms. Influenza B tends to spread in a specific area or region, causing an epidemic, which is a class below pandemic. This strain also mutates less quickly, so limited immunity is possible, though not lifetime immunity. Fewer people are infected with influenza B simply because it is less common. Like all three strains, influenza B causes symptoms such as a fever, chills, nausea, nasal congestion and others.
Strain C
Influenza C is the least common variety of the three strains. The CDC states its researchers do not believe this strain causes widespread infection or epidemics. It typically occurs in children and produces only mild symptoms.
