Vaccines induce protection against infections by
stimulating the development of long-lived effector cells
and memory cells. Most vaccines in routine use today work
by inducing humoral immunity, and attempts to stimulate
cell-mediated immune responses by vaccination are ongoing.
The success of active immunization in eradicating
infectious disease is dependent on numerous factors.
Vaccines are likely to be must effective against
infectious that are limited to human hosts and are caused
by poorly infectious agents whose antigens are relatively
On the other hand, antigenic variation, the existence of
animal or environmental reservoirs of infection, and high
infectivity of the microbes make it less likely that
vaccination alone will eradicate a particular infectious