Flu is caused by a virus. Other animals such as pigs and birds can also be infected by a pig or bird version of the flu. When pigs, birds and humans all live close together, ther is a chance that human flu, pig flu and bird flu can all be in the same person at the same time. Normally this is fine until the flu viruses start to swap genetic information. The new virus then has bits of each of the old ones which allows it to jump species.
Swine flu is the result of a pig virus becoming humanised in that it can infect humans. This type of flu is often more severe than seasonal flu because its new and non of us have seen it before. We have no immunity to it, our immune system which usually fights off infections takes time to get going on the new virus and for some people they don’t survive.
Flu can affect the lungs and cause them to fill up with fluid and eventually blood which causes the patients to die. The flu does this so it can go and infect someonelse and start the process all over again.
Keeping hands clean can help and limiting exposure, and there are a couple of drugs which help and a vaccine for te seasonal flu.
Most healthy people are ususally okay, but if you have an underlying disease, you may find it more difficult to fight off the infection.