Vaccines help protect against specific infections by bacteria or viruses. They are administered by a health care provider (nurse or doctor) by injection, nasal spray, or a pill taken by mouth. Vaccines usually contain small pieces of the bacteria or virus in a weakened but live or killed form. For instance, the varicella (chickenpox) vaccine is an injection containing very weakened particles of the chickenpox virus. The virus in the vaccine is not potent enough to cause an infection in most cases but it can stimulate the immune system to make antibodies. These antibodies help protect the person against future infection.