A vaccine harnesses the body's natural ability to defend itself. Vaccination (or "immunisation") stimulates an immune response that will protect your pet from the natural form of a disease. Vaccine manufacturers either kill micro-organisms, modify living ones so that they don't cause clinical illness or use genetic engineering to instruct harmless bacteria to manufacture bits of virus particle useful for stimulating the body to produce antibodies against it. These modified microbes, or parts of microbes are introduced into your pet's system as "vaccines". Most are given by injection but some, for example Bordetella vaccine to prevent kennel cough, are given by aerosol in the nose.