Vaccination has been one of the most important interventions designed to prevent disease to be employed on a worldwide basis, second only to the improvement of sanitation services and the provision of clean drinking water. Live virus vaccines use the weakened (attenuated) form of the virus. The measles, mumps, and rubella (MMR) vaccine and varicella (chickenpox) vaccine are examples. Killed (inactivated) vaccines are made from a protein or other small pieces taken from a virus or bacteria. Vaccine approaches to infectious diseases are widely applied and appreciated. Amongst them, vectors based on recombinant viruses have shown great promise and play an important role in the development of new vaccines. Many viruses have been investigated for their ability to express proteins from foreign pathogens and induce specific immunological responses against these antigens in vivo. The theoretical problem of the effects in man of viruses that are oncogenic in rodents and are derived from various tissue culture systems deserves serious attention. However, this consideration, that of antigenic potency, and other problems reviewed should not be allowed to subvert efforts to solve the real problems that face us, the disability and death resulting from these common infections.