About Influenza Viruses

Influenza viruses are infectious agents that can sicken many people at once, as happens in a pandemic or seasonal flu epidemic. There is no cure for a virus, so prevention is key.

Three Types

• There are many strains of flu, but all can be categorized into three types: A, B and C. Type C is mild, and A and B cause seasonal flu, with A also responsible for pandemics.

Avian Flu

• Birds carry all influenza A viruses. Those limited to bird populations---as opposed to those humans already have---are called "bird flu" or "avian flu." Bird flu only rarely transfers directly to humans.

Swine Flu

• In pigs, human, bird and swine viruses can combine to create new strains of flu, as occurred with the 2009 H1N1 influenza.

Canine Flu

• Dogs get canine flu caused by an influenza A virus that first infected horses. After this H3N8 equine virus jumped to dogs, it adapted allowed them to spread it to one another.

Subtypes

• Influenza A can be subtyped based on two proteins found on the virus: H.A. (hemagglutinin) and N.A. (neuraminidase). Thus, H1N1 indicates a virus that has both an H.A. 1 and N.A. 1 protein.

Strains

• Besides types and subtypes, viruses have strains produced by antigenic drift, whereby small changes occur in a virus over time. Such drift requires flu vaccines to be updated annually.