Introduction

Plague has been intensively studied. These efforts have produced a vast amount of microbial, medical and epidemiological research. Most of it is synthesized in outstanding standard works on plague. Here, only the outline and central findings of this research can be presented and updated. This presentation is amply footnoted to permit and encourage further reading.

Lethal doses of plague bacteria

The basic conditions of plague mortality must be considered. This study can usefully begin with the number of plague bacteria which, transmitted to humans, will constitute a lethal (mortal) dose because it clears the ground for other basic questions relating to sources of infection and mechanisms of epidemic transmission. Infectious doses of bacteria are designated ID, lethal doses LD, for, for instance, 50% of an infected normal population LD50.

For evident ethical reasons, experiments cannot be performed on humans to ascertain LDs of plague infection. Because monkeys have proven unsuitable, important clues must be based on rodent studies, and, unfortunately, these are few. Importantly, 90% of black rats (Rattus rattus) survived primary subcutaneous inoculation with 5,550 plague bacteria (= LD10), LD50 would obviously require a substantially higher infectious dose. The LD50 of a species of California ground squirrels that generally is considered highly susceptible to plague, is 6,070 bacteria. These data have limited relevance for the question of LD50 for human beings, except indicating the minimum infection that could result in this mortality rate. Humans are, apparently, quite susceptible but not highly susceptible to plague infection. This is also reflected in the fact that 20% of plague diseased in normal populations in early developing countries and in historical societies survived (Benedictow 1992/1993/1996: 146–9). Arguably, the use of ~6000 Y. pestis as indication of LD50 for a normal population in developing countries or in historical societies is a cautious choice of analytical tool.

The size of blood meals ingested by fleas and by lice

This clarification of LDs of plague infection for humans immediately raises questions relating to humans and rats as sources of infection with plague bacteria.

• 1. It is often mechanically and arbitrarily assumed that (black) rats and humans function about equally as sources of infection of feeding ectoparasites.