The changing pattern of infectious disease.

Several factors contribute towards a decrease in the prevalence of infectious disease in a population. These include active control measures, active immunisation, and improvement in the socioeconomic state of the population. There appears, however, to be a progressive increase in the resistance of a population in relation to the length of time the population has been exposed to an agent. This increasing resistance is currently thought to be an expression of natural selection but transmission of actively acquired immunity cannot be ruled out and in the light of current evidence remains a highly probable contributory factor.