Live attenuated vaccines (e.g., chickenpox [varicella] vaccine, measles-mumps-rubella [MMR] vaccine), which use live, weakened viruses, may cause a child to develop a very mild form of the disease.In most cases, the illness is less severe than infection with the actual disease.mmunizations, also called vaccinations or inoculations, protect children against more than a dozen serious diseases that can cause severe illness, lifelong disabilities, or even death.