Influenza types

The normal circulating human strains of influenza are generally known as "seasonal influenza". The origin of the viruses is thought to be birds, which are the only species to be infected by all the different subtypes of influenza viruses. But influenza virus can infect a wide range of animals, and several species are more susceptible to particular subtypes of influenza virus. Human influenza A strains tend to be H1 or H3, while other species have different H or N markers.

Influenza strains that infect pigs have been called "swine flu", and strains that infect birds are known as "avian influenza", or "bird flu".

Equine (horses) influenza is a serious infection of those animals, while canine (dogs) influenza is a troublesome vetinenary problem. Cats and even whales can be infected with their particular strains.

When a new virus appears to be able to infect humans, this is called "pandemic influenza" The new strain that is circulating at present is now known as Pandemic (H1N1) 2009 strain.