Vaccines work by mimicking a natural infection without causing disease. Because the body is now familiar with the infectious agent and knows how to fight it, the immune system is primed and ready when the real infection is encountered. First, the infectious agent must be separated and identified, and then work can begin on developing a vaccine. If the agent can not be isolated then vaccine development is virtually impossible.There are three types of vaccines: live vaccines, inactivated vaccines, and synthetic vaccines. Live vaccines are produced by growing the infection causing material under carefully controlled conditions that limit their virtulance. Usually one vaccine injection is sufficient to produced the desired immunity. Inactivated vaccines are produced by killing the infection causing micro-organisms with chemicals or heat. However, most inactivated vaccines stimulate a somewhat weak immune response and require future booster shots to maintain immunity. Synthetic vaccines can be developed from antigenic fragments via genetic manipulation. The fragments of the original material are inserted into a carrier which when injected into a human can often evoke an immune response.Infectious agents are constantly changing because they are capable of evolving through natural selection and mutation.Viruses evolve to evade the immune system, and can do so very quickly. A virus that is not suspectible to a particular immune system response will survive and replicate, passing along the ability to survive.