Vaccination in thoroughbreds reduces the susceptible population and lessens the likelihood of largescale outbreaks.

Use of vaccination programmes before the introduction of an eradication campaign can reduce the susceptible population.

As a pathogen that confers immunity to the survivors moves through a susceptible population, the number of susceptible contacts declines.

As prevalence decreases, however, the incentive, and thus demand, will slacken and allow the susceptible population to increase until the disease can reinvade.

In addition, the differences between individuals due to genetics or other factors mean that the hazard may be higher for particular groups, called susceptible populations.