The two ways to achieve immunity are actively and passively. Active immunity is achieved when exposure to a foreign stimulus triggers an immunological response to the agent by the host. The best immunity to an agent is achieved by natural infection, which evolves with a clinical or subclinical response to the agent by the host. Artificial active immunization is the administration of an immunogen as a vaccine. Vaccines may be live organisms, killed organisms, or modified toxins. Although no vaccine is ideal and each has its problems, the problems of live vaccines are generally related to their safety, while the problems of killed vaccines are related mainly to their effectiveness.