We usually talk of viruses being 'species specific' - which means that they make only one species (human or pig or chicken, for example) sick

but if an animal such as a pig, which lives near chickens and humans, is unlucky enough to be infected with viruses which attack chickens and humans too, then there is a small chance that the DNA from the pig, chicken and human viruses will mix together and produce a new virus that has never existed before...

... and because most humans do not live near pigs, they have not built up good immune defences against pig viruses, so this new virus can make them very, very sick indeed!

Our immune system needs to "see" an antigen , which can be a protein on the outside of a virus' protein coat, in order to protect us by making cells producing antibodies and "sounding alarm bells" for other cells of the immune system to attack the virus.

Once we have seen an antigen once, some immune cells act as memory cells , so that our bodies "remember", and can produce many more antibodies if we ever meet the same antigen again.

So people who work with pigs probably have a good immune system to protect them from the new viral strain, because their immune system has "seen" many pig antigens before, so the immune cells will mount a big attack which will save their lives if they catch swine flu.

More details of how the immune system "sees" an antigen and protects us here

Action:

Unfortunately the virus is the one doing all the action at the moment - it is passing from human to human and causing great concern.

So, the most important thing is for us all to make it difficult for the virus to spread - watch the news in your area to find out how to do this!

Meanwhile, it is always a good idea to stop bacteria and viruses spreading by:

a)  washing your hands often with soap and water

b)  cough into a tissue to stop the droplets containing bacteria or viruses spreading all around

c)  avoid close contact with other people

d)  only use your own eating knives or cups etc, don't share other peoples', even if you are good friends

but if you do get sick, rest and tell someone so that you can get help to get better again as quickly as possible!!!

Play an online game to learn about the immune system here!