A DEAD RAT, found a few days ago not far from President Reagan's ranch in California, turns out to have been the victim of bubonic plague. It's the sort of thing that the Secret Service takes seriously. Who let that rat in here? Does it mean that the Age of Reagan is going to take us back not just to the time of Calvin Coolidge, but actually to the 14th century? The Democrats, as we all know, are much more advanced than that--at least by several centuries. We put them up to the age of Defoe, and suggest that so far as Democratic history is concerned, 1981 might easily qualify for the Journal of a Plague year.

The rat was native to the place, but the plague bacillus came by a longer route. It's en demic throughout a broad band of western North America, from Canada down into Mexico, among rodents that can, of course, transmit it to people. If you are ever bitten by a rat, or a squirrel, or a gopher, especially in the West, you ought to take yourself promptly to a doctor.

The plague wiped out perhaps half the population of Europe when it first arrived in the middle of the 14th century. But over the years the succeeding epidemics gradually lost their force; the last important one in Western Europe was in the early 18th century, in Marseilles. Perhaps the bacillus itself changed. Certainly rising standards of living increased human resistance to it. Quarantine procedures im proved. The plague did not arrive in California by the Atlantic route, but by the Pacific.

It migrated from central Asia slowly down through China during the late 19th century, in the wars and internal chaos that accompanied the disintegration of the Manchu rule. Disease always travels most rapidly in the company of other miseries. The plague arrived in Hong Kong just before the turn of the century, and from there jumped rapidly to seaports all over the world. When ships moved by sail, infected rats were usually dead long before voyages ended. But steamers, moving more rapidly, were able to cross even the Pacific before the disease had run its course. As Asian rats came ashore in California, the bacillus spread to other rodents.

American ranchers then deliberately spread the plague across the plains, in an early example of ecological warfare. When a steer steps into a gopher hole and breaks a leg, the value of the steer drops to zero. The ranchers were trying to use the plague to exterminate the gopher, in one of the less attractive episodes in the development of the West. They succeeded only in ensuring that the bacillus would live there happily ever after.

If you had contracted bubonic plague 60 years ago, you would have had less than an even chance of survival. Today the cure is safe and rapid--if, but only if, you have access to antibiotics. Bubonic plague sticks in the collective consciousness of humanity as a symbol of the unimaginable disasters that it has survived, and of the losses that they have inflicted. The dead rat is also a reminder that the world remains full of dangers from which even a diligent Secret Service cannot provide complete protection.