When bubonic plague ravaged Europe in the 14th century, it was spread by rodents swarming through the squalid housing of the poor. Now plague is on the increase in the western United States because of the living habits of the affluent.

As Americans seek solitude in nature, building houses in more primitive settings away from the cities, they are finding they have to put up with primitive dangers, including bubonic plague.

Eight cases have been counted this year, three of them fatal, and the high season for plague continues through October.

The eighth victim was found yesterday at a ranch on New Mexico's Sandia Mountain, the second case in a week in the area.

From the 1920s until the late 1960s, the United States counted an average of one human case of plague a year, said Jack Poland, an expert in bubonic plague at the Centers for Disease Control in Fort Collins, Colo. But since 1975, he said, the average has been between 15 and 20 cases a year, all in the western states.

Plague is no longer the great threat to life it once was. It now can be cured quickly with antibiotics if caught before advanced stages. But because it is sometimes hard to diagnose, deaths occur annually and health departments in the West routinely check wildlife for signs of the disease.

The disease, which earned the name "the black death" when it wiped out as much as half the population of Europe in a few years during the 1300s, arrived in America at the turn of this century and has existed among rodents here ever since.

In the early 1900s it was learned that plague was caused by a bacterium that infects fleas. When the infected flea bites an animal and feeds on its blood, plague bacteria is passed into the victim. Some animals are relatively immune to the bacterium and carry the fleas until they die. Then the fleas seek a new host -- sometimes man.

Since the method of passing the disease was learned, it has been held under control in humans. The disease is still common among some rodents, but man rarely has come in contact with the chief carriers -- the rock squirrel, the ground squirrel and other western rodents.

"But as people move out farther from the city, and build these beautiful rock walls, where the rock squirrels love to live, and wood piles and other things, then you have people coming into contact with the disease again," Poland said.

Among the rich who have moved to more primitive surroundings are Ronald and Nancy Reagan, whose mountain ranch retreat is outside Santa Barbara, Calif. In May a live, plague-infected wood rat was trapped less than a mile from the Reagan ranch house.

Since the Santa Barbara Health Department doesn't often find infected rodents, the local citizenry and the president, who dislikes field mice and sets traps for them, were warned to take precautions, such as tucking jeans into boots and refraining from handling wildlife.

Ross Grayson of the Santa Barbara county health department said that results of a recent routine rodent-trapping are not known yet but that it netted few rodents.