A vaccine is a biological preparation that establishes or improves immunity to a particular disease. There are multiple types, each helping the human immune system in different ways.
Vaccines have contributed to the eradication of smallpox, one of the most contagious and deadly diseases known to man. Other diseases such as rubella, polio, measles, mumps, chickenpox, and typhoid are nowhere near as common as they were a hundred years ago. As long as the vast majority of people are vaccinated, it is much more difficult for an outbreak of disease to occur, let alone spread. This effect is called herd immunity. Polio, which is transmitted only between humans, is targeted by an extensive eradication campaign that has seen endemic polio restricted to only parts of four countries. The difficulty of reaching all children as well as cultural misunderstandings, however, have caused the anticipated eradication date to be missed several times.
Opposition to vaccination, from a wide array of vaccine critics, has existed since the earliest vaccination campaigns. Disputes have arisen over the morality, ethics, effectiveness, and safety of vaccination. The mainstream medical opinion is that the benefits of preventing suffering and death from serious infectious diseases greatly outweigh the risks of rare adverse effects following immunization.