Live attenuated vaccines contain a form of the living organism that has been weakened so it should not cause disease. There is the possibility that the living microbe in the vaccine could become virulent and cause disease. Examples of live attenuated vaccines are intransal influenza vaccine, varicella vaccine, oral polio vaccine, and the measles, mumps, and rubella vaccine. The potential of attenuated microbes to become virulent is of concern related to prevention of infection among patients with cancer, particularly those who are significantly immunocompromised.