T he plague bacillus can go anywhere. But it will travel faster and further in the aggravated conditions known as septicaemic or pneumonic plague, when direct person-to-person infection can take place. In those extreme but rare variants of bubonic plague, both probably present in the Black Death of 1348-9, plague bacilli may concentrate in the bloodstream in such huge quantities that they can be carried by the human flea (Pulex irritans), or transmitted by coughed-up droplets and blood-bearing sputum. Such conditions, however, have remained exceptional. And it has always been more common for plague to be carried by the rat flea (Xenopsylla cheopis), and to be limited in range by the black rats usual run of not more than 200 metres. Other rodents subsequently spread the plague bacilli much further, especially in close-knit communities such as towns. But, except in grand pandemics like the Black Death itself, bubonic plague has stayed throughout its history primarily urban-based, seldom reaching out with full force into the surrounding countryside, and killing many more in the towns than in their hinterlands.1