To date, the avian flu virus has not acquired to ability to spread easily from person to person – a necessary step in order for a virus to cause a pandemic.
Other combinations circulate in animals, such as the H5N1 virus found in birds.
Every few decades or so, a new version of the influenza virus emerges in the human population that causes a serious global outbreak of disease called a pandemic.
Pandemics are associated with widespread illness - and sometimes death - even in otherwise healthy people.
Influenza pandemics are caused by type A viruses, and therefore these are the most feared type of influenza virus; neither types B or C have caused pandemics.
Type A influenza is classified into subtypes depending on which versions of two different proteins are present on the surface of the virus.