Flu Type A vs. Type B

Lately, I find the flu capturing my interest. For a long time, I thought the flu was just the flu. Just another virus going around. But it turns out that there’s more than one type of flu… WAY more than one.

The influenza virus is divided into three types:

  • Type A: the most common version of the flu. It is also the most serious — the one that caused flu epidemics throughout history. Influenza A can infect people, birds, pigs, horses, and other animals.
  • Type B: a milder version of the flu. Also to blame for epidemics in the past, but not quite as deadly as Type A. Influenza B generally only appears in humans.
  • Type C: more like a mild cold than a true flu. Has never been blamed for a large epidemic.

Because influenza A is the Big Bad Wolf of flu viruses, it is the only one with sub-types. These sub-types include low pathogenic (mild) and highly pathogenic (severe) avian flu viruses. Sub-types are divided based on different proteins on the surface of the virus.

The flu virus evolves in two ways: antigenic drift and antigenic shift. Antigenic drift means the small, gradual changes that occur in a virus. There are two genes in a flu virus that contain the genetic material used to produce surface proteins. Mutations in these genes can produce new strains of a virus — strains that may slip past antibodies that attack other versions of the same virus. These new strains are often named for the area in which they developed or were first recognized.

Antigenic shift is a sudden, major change to create a brand new influenza A sub-virus. This can occur through direct virus transmission from animal (poultry) to human or when human influenza A and animal influenza A mix and create a new virus. Flu pandemics are often the result of antigenic shift — a totally new sub-type appears in the human population and spreads quickly from one person to another.