H5N1 or avian flu is a virus that is predominantly associated with causing infection in birds.
Virus are “organisms” that require a host to replicate. This virus has its own RNA replicating enzyme. This enzyme lacks proof reading capabilities. That is to say if the enzyme makes a mistake and pairs up the base pairs incorrectly that mistake will go unnoticed and will be in the RNA of the next virus. So each time the virus replicates a single mistake is made. Over time these mistakes/mutations can add up and alter the behaviour of the virus such as switching from infecting birds to infection humans.
The RNA molecules in H5N1 are also segmented into 8 separate entities. This means you can get genetic exchange by segment reassortment in hosts co-infected with 2 different influenza viruses. Another great way to cause mutations.
Influenza is an RNA virus – meaning its genome is made up of ribonucleic acid, rather than DNA (deoxyribonucleic acid). It replicates using an enzyme called the ‘RNA-dependent RNA polymerase’, which allows fast copying, but due to a lack of proof-reading (or spell-checker), is also error prone. We call these errors mutations. Some of them won’t survive, as they’ll change viruses ability to function for the worse, but lots be will be carried on to the next replication, when more mutations can occur.
If you’ve got access to youtube, you might find this video helpful to watch: https://www.youtube.com/watch?v=X3dSYA64ZRc. It talks about swine flu (from pigs), but the same principles apply to avian influenza as well.