People can get diseases or injuries from their environment. Some of the causes of these can be easily seen, for example, an injury from slipping and falling on a slippery floor, or cutting a foot by stepping on broken glass.

For some diseases the causes are not so easily seen. There are many diseases which come from animals that are so small they cannot be seen without the help of microscopes, which make them look much larger than they really are. These tiny animals are germs and parasites. They can cause diseases such as colds, diarrhoea (runny tummy), hepatitis A (liver disease), skin infections and anaemia (weak blood).

Some of these diseases or injuries can be very serious and even cause death. Some are not serious at all - they are just annoying.