The Causes Of Influenza

Influenza (flu) is a respiratory infection caused by certain viruses. It is caused by the influenza type A, B, C or D viruses. A and B are to blame for annual flu epidemics that can reach up to 20% of the population. Influenza is linked to 3,000 and 49,000 deaths per year in the United States as well as 200,000 hospitalizations. Flu shots are based on the most predicted active strains in the year and generally includes up to four strains. Worldwide epidemics can lead to one billion cases of the flu and 250,000 to 500,000 deaths per year.

Influenza is spread by sneezing, coughing and by speaking. When particles of the virus are released into the air they then can enter the mucus membrane via the nose, mouth or eyes. To help prevent the flu it is therefore important to wash the hands frequently or get vaccinated with annual flu shots. Infection can happen if a person touches objects that have been contaminated by infected droplets such as doorknobs, television remotes or someone’s hands and so forth. Incubation period is generally two days but can be in the range of one to four days.

Type A:

Influenza A is a virus that can infect animals as well although more common in humans. Type A viruses are divided into two subtypes based on two surface proteins: haemagglutinin (HA) and neuraminidase (NA). There are eighteen of the subtype haemagglutinin (HA) and eleven of the subtype neuraminidase (NA). Of these, the viruses that have been most active have been 3 subtypes of HA (H1, H2 and H3) and 2 subtypes of NA (N1 and N2). They are then further broken down into specific strains such as H1N1 and H3N2. Wild birds are often seen as hosts for this Type A but not always. There are three primary avian flu subtypes: H5, H7 and H9 with H5 and H7 being the most dangerous. Type A constantly changes and is usually the one we look at for the larger influenza epidemics.

Type B:

Type B is exclusively found in humans. This type may be less severe than Type A, they are not classified by subtype and they do not cause pandemics. The two lineages that have developed are: B/Yamagata/16/88-like viruses and B/Victoria/2/87-like viruses

Type C:

Influenza C is also only found in humans. It is milder than both Type A or B and people generally do not become very ill with Type C. They do not cause epidemics.

Type D:

Not known to infect humans and generally seen in cattle.


When reported they are named as follows:

  • The antigenic type (A, B, C)
  • The originating host animal (swine, duck, chicken, etc.)
  • Geographical origin (Boston, Edmonton, etc.)
  • Strain number (35, etc.)
  • Year of isolation (1976, etc.)
  • For Type A viruses, the hemagglutinin and neuraminidase description ((H1N1), (H5N1))

Which would end up looking like: A/duck/Alberta/35/76 (H1N1) for an example of a virus with a duck host origin.
The best protection from Type A and B is your annual flu shot and to wash your hands regularly. Most people recover from the flu within 7-10 days, but in others it can get severe and even require hospitalization.