Herd immunity
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Herd immunity describes a type of immunity that occurs when the vaccination of the a portion of the population (or herd) provides protection to un-vaccinated individuals.
The effectiveness of a vaccine depends on the immune system reaction which the patient develops. This involves the generation of long term memory B cells and T cells via adaptive immunity following innate immune responses. Sometimes the antigen contained in the vaccine doesn't trigger an immune response. In the latter case there is need for new and stronger vaccines.
Vaccination acts as a sort of "firebreak" in the spread of the disease, slowing or preventing further transmission of the disease to others. Although no vaccine offers 100% protection, the spread of disease from person to person is much higher in those who remain unvaccinated. Virologists have found that when a certain percentage of a population is vaccinated, the spread of the disease is effectively stopped. This critical percentage depends on the disease and the vaccine, but 90% is not uncommon.
It is the general aim of those involved in public health to establish herd immunity in most populations. Some argue that vaccination protects only vaccinated persons from pathogens. However, wide spread vaccination is not always possible, and vaccines do fail. (See the MMR vaccine controversy in the UK.)
Herd immunity should not be confused with contact immunity, a related concept wherein a vaccinated individual can 'pass-on' the vaccine to another individual through contact.