Vaccines may act in two ways, by increasing the immunity of an individual and by increasing herd-immunity. Vaccines against poliomyelitis and diphtheria have been extremely successful for both purposes. Other vaccines are now being introduced against diseases with less morbidity and mortality, and it is important that these vaccines are safe as well as effective and that they do not lead to long-term sequelae. Work on zoster and, more recently, on subacute sclerosing panencephalitis and other diseases has shown that some viruses may persist and cause disease after a long latent period. If a disease is eventually acquired by most people, the administration of a live vaccine is an alternative method of experiencing the virus. This may avoid the unpleasantness and dangers of the natural disease, but the relative risks of late sequelae following natural infection or vaccine are as yet unknown.