Immunity as a factor in the epidemiology of medieval plague.

The rise, disappearance, and demography of medieval plague remain mysterious. This paper reviews those features of plague immunity in animals and humans that might help explain these aspects of medieval plague. The absence of a focus of sylvatic plague in medieval Europe is postulated, while it is suggested that the susceptible population was much more varied both in terms of immunity and number of species than has been assumed. The sex, age, and regional variations in incidence of plague among humans are examined on the basis of primary and secondary historical sources. Nutritional factors, especially iron status, and cross-immunities with other diseases help provide an explanation of the observations in the literature.