There are 3 types of flu viruses that affect humans:

Type A – usually the more serious type, responsible for yearly outbreaks and pandemics.

Type B – tend to cause less severe disease and smaller outbreaks affecting mostly children.

Type C – causes mild illness similar to the common cold.

A fourth type of virus, known as D virus, exists but this primarily affects cattle and is not known to infect or cause illness in humans.

Each of these types have different groups within them that will refer to as strains. In the media you may see different strains referred to using their scientific classifications e.g. H1N1, H5N1