Amid the startling rise in measles cases it is imperative to draw upon a basic tenant of public health: herd immunity. This is the relative protection provided to unvaccinated individuals by being surrounded by those who are vaccinated, and hence less likely to transmit infectious agents to the unvaccinated one.
In the above scenario on the far left, the person in red color who is ill from measles (or another contagious disease) is surrounded by few vaccinated persons in blue color. Thus, moving next image to the right, this one unvaccinated person can rapidly disseminate the disease, and soon all the blue, unvaccinated persons become red and ill.
By contrast, in the right-side scenario, the ill person in red color is surrounded by many vaccinated persons in yellow color, thus protecting those persons who are unvaccinated in blue color.
One important implication of heard immunity is the imperative that everyone be vaccinated, and that no one simply assume that the vaccinated persons surrounding them will provide protection. For herd immunity to be effective, everyone must be part of the herd.