Flu viruses can infect lots of different animals. The animals that flu viruses infect depends on the combination of HA and NA proteins (receptors) that it has on its surface. Flu viruses need HA to get into cells and NA to spread to other cells. This is why they are often used as targets for anti-flu drugs.
There are different sub-types of HA and NA receptors, and scientists number them, like H1N1 and H5N1. Some combinations of HA and NA receptors can infect many kinds of animals.
Pandemics are caused when flu viruses with a new combination of HA and NA types evolve and spread. The 1918 Spanish flu and 2009 swine flu pandemics were both caused by H1N1, but one was much more devastating than the other. This is a result of the different abilities of the virus to cause disease (virulence) and the people to fight their infections (immunity).
Click on the button to find out more about the ‘Flu Fighters!’ activity that I was involved in developing for the Edinburgh International Science Festival.