When used appropriately, immunization can effectively prevent many infections and diseases. Some vaccines, such as that for polio, are believed to produce lifelong immunity. Others, such as those for tetanus and diphtheria, may require that a booster injection be given upon exposure to assure full immunity. Still others, such as that for influenza, confer immunity for only a limited time. Inoculation is not without risk, particularly in immunosuppressed, allergic, febrile, or pregnant patients. However, in otherwise healthy patients, serious sequelae are so rare that they are far overshadowed by the enormous benefits of immunization.