State health department officials are monitoring a rabies epidemic which is spreading throughout the state's raccoon population, afflicting many of the animals and posing a threat to humans.

The disease, which has migrated northward from the Appalachian Mountains, has been confirmed in raccoon populations in nearly 90 percent on Massachusetts towns in the past year, according to state disease control records.

"We had a high population of raccoons in the state, so when it entered, the virus was able to spread rapidly," said Pamela R. Landry, rabies specialist with the Massachusetts Department of Fisheries and Wildlife.

And since raccoons are attracted to high density areas like Cambridge, which provide an ample supply of food and residential garbage for the hungry critters, residents take special precautions when they venture outdoors, said Sean M. Fitzpatrick, spokesperson for the state Department of Public Health.

Most humans contract rabies after being bitten by their domestic pet which had been attacked by a wild animal earlier, according to Mark W. McGabe, director of the Cambridge Animal Commission.

"The best protection is to make sure that your domestic animals are vaccinated," McGabe said.

Residents should also avoid stray or wild animals, keep the lids on their trash cans securely fastened and contact animal control officials if they see animals who appear to be rabid, Fitzpatrick said.

Human beings who acquire rabies must undergo a series of six vaccination shots which cost about $3,000, he said.

"These simple precautions generally will keep somebody free from an expensive and painful ordeal," Fitzpatrick said.

Because the disease kills ailing animals within one week, the Boston area's raccoon population has declined by as much as 80 percent recently, Landry said. But there is little danger that the species will become extinct, she said.

But the rabies virus also will likely continue to plague the raccoon population, she said.

"It won't go away," Landry said. "As the disease runs its course, we'll see a reduction in cases, but it will always be in the population."

Although few cases of rabid animals have been reported in Cambridge, McGabe said he is looking to Cape Cod officials for possible help in designing prevention measures.

Officials there are trying to halt the spread by sprinkling vaccine laced food near raccoon habitats on the island, he said.

"It's a more aggressive approach," McGabe said.