Most vaccines are given by injection under the skin. A few vaccines may be given as a spray up your cat's nose. They all work by training the white blood cells in your cat's body how to recognise and attack the viruses or bacteria contained in the vaccine. This should prevent infection with that particular bug if your cat is in contact with it again. Current vaccines fall into two main categories:

• Live vaccines: these contain a strain of the bug which has been altered so that it cannot cause disease but does stimulate immunity
• Dead vaccines: the bug in these has been killed by heat or chemicals

Each type has their pros and cons - live vaccines generally give better and longer-lasting protection but they can sometimes cause more side effects. Live vaccines are not recommended for certain groups of cats, such as pregnant females.