Luckily our immune system is quick to detect a foreign protein, like the ones on the outside of viruses, and mount an attack... and even remember it for the future!

So as we get older we are likely to build up a good immunity against viruses causing flu, which tend to be rather similar.

But every now and again, a new strain of virus evolves - see yesterday's news item for details of how the swine flu that has swept Mexico emerged as a combination of DNA from different viruses, with the result that it can now infect humans and be transmitted from one human to another.

Protein molecules on the outside of the virus are what help it to cause us harm, and they are used to name the type of virus too:

for example viruses often have the initials H for haemagglutanin, which helps the virus stick to the surface of cells it will attack and N for neuraminidase, which helps the virus be released from the cell it attacked and infect new ones.

Human viruses with these two proteins are therefore more dangerous!

The H1N1 strain that caused the 1918 flu pandemic was possibly the worst of all. Since then it has mutated (changed its DNA) - read about the different strains it formed and the pandemics they caused in the article.

Action:

Keeping up the preventative measures, such as washing hands regularly, and using a tissue when you cough or sneeze, is good all year round, not just in the flu season!