Wild plague exists in natural foci independent of human populations and their activity. Domestic plague is intimately associated with rodents living with humans and can produce epidemics in both human and animal populations. Humans are extremely susceptible to plague and may be infected either directly or indirectly. Indirect transmission through the bite of a flea is the most common route of transmission between plague-infected rodents and humans. Human infection can occur within the natural foci of plague but this is rare.
Infection occurs more frequently in human settlements when domestic rodents become infected.
This can happen when the domestic rodents come into contact with infected wild rodents living in the surrounding areas. Infected fleas leave the bodies of rodents killed by plague seeking a blood meal from another host and may bite human beings. In the last two millennia, plague has become widespread, affecting a large number of countries on most continents during several pandemics. The first pandemic that we are certain of, known as Justinian s plague, occurred between 542 AD and 546 AD, causing epidemics in Asia, Africa and Europe. It is estimated to have claimed nearly 100 million victims.