Plague is usually a disease of wild animals. It progresses through a cycle in which the bacterium infects fleas, the fleas then infect animal hosts and, finally, both fleas and animals die of plague. But before they die fleas often transmit the plague to other hosts-usually animals but sometimes humans.

When humans become infected, an epidemic may ravage a whole community. Sometimes entire families perish before medical science can detect the bacillus and prescribe a successful antidote. More often than not the carnage takes place unnoticed among wild rodents.

On rare occasions conditions grow ripe for plague to breach the boundaries of the big cities. When this happens the disease explodes within the midst of a tightly packed population. The common rat is the major factor in transporting prairie plague into the cities.

Ironically, the seemingly inexhaustible reservoirs of wild-animal plague seldom include the rat. When it comes to the plague and its effect on human beings, the common domestic rat is the king of culprits. It is the deadly messenger that carries the plague from infected wild animals to the human environment. History bears out that the rat can pass the deadly disease over thousands of miles: from central Asia or Africa into India, Southeast Asia, Europe and, in the early part of this century, into the Americas.

The black rat and the Norway rat play dominant roles in plague transmission. Their abilities to live off the human habitat are legendary. Rats gnaw incessantly; their incisor teeth grow at least five inches a year. They can penetrate materials that almost defy imagination: lead pipes, cinder block, concrete that has not completely hardened, plastic, fiberboard, asbestos and aluminum siding.

They can squeeze through a hole as small as the size of a human forefinger. Climbing trees and pipes is a simple feat, and they can scamper full speed along telephone wires using their long tails to maintain balance. Rats are incredibly prolific and destructive.

Rats are forced to migrate when overcrowding, predation, starvation or disease threatens their survival. Circumstances determine their migration. In South America the migration of black rats relates directly to the ripening and decay of a dominant species of bamboo. When unusually large quantities of bamboo seeds ripen, the rats multiply rapidly. When they don't, the hungry rats descend on cultivated areas, destroying and consuming crops.

The World Health Organization estimates the world's rat population to be at least four billion. But let's not forget the common mouse. Its population is estimated to be as large or larger than that of the rat. As one professor of environmental studies put it: "Frankly, I'm more concerned about mice than rats. The rat may be in your basement but the mouse will be in your cereal box."

The rat population in the United States is estimated at between 100 million and 200 million. New York is estimated to house six to eight million rats.

Some rats within the contiguous United States have been found to be plague-infected. Plague exists, and its most dangerous messengers are rats. "We depend, for the prevention of catastrophe, upon approximately equal measures of eternal vigilance and continued good fortune" (Charles T. Gregg, Plague: An Ancient Disease in the Twentieth Century, University of New Mexico Press, Albuquerque, 1985, p. 72). GN