How does herd immunity reduce the spread of a contagious disease?

Herd immunity happens when a large enough share of a population has immunity that spread becomes difficult. The exact threshold depends on how contagious the disease is; measles, for example, requires very high coverage because it spreads easily. Immunity can come from vaccination or prior infection, but vaccination is much safer than relying on disease spread. Herd immunity is one reason vaccination programs can protect infants, people with weakened immune systems and others who are vulnerable.