NEW YORK -- The blight spread with the swiftness of an oil slick, first fouling the south shore of Long Island, then the New York City beaches, then part of the New Jersey shore. By last week, it had spread as far north as Connecticut, Massachusetts and Rhode Island and as far south as Maryland.

It has left behind polluted waters, deserted coastlines and a growing sense of unease. Indeed, it has often seemed during this sweltering summer of 1988 that all of society's problems -- from drug addicts' needles to AIDS-tainted blood, from crack vials to nonbiodegradable plastic bags, from raw sewage to dead rats -- have come home to roost along the beaches of the Northeast.

Despite a frenzied search for the hidden culprit or culprits, many officials here now say they believe that the beach closings have been caused by the same nagging pollution problems they have been grappling with for decades.

"Everyone wants to get that magical hold on something and fix it and have it all go away," said William J. Muszynski, deputy regional administrator for the Environmental Protection Agency's New York office. "But it's a combination of events."

Those events include heavy rains that wash untreated waste into the ocean from underground sewer pipes; breakdowns at overloaded and outmoded sewage treatment plants; garbage barges that lose some of their cargo while sailing toward towering landfills; hospitals and laboratories that illegally toss medical waste in the trash, and factories that discharge toxic waste into the water.

"The garbage, the needles, the medical waste are just the tip of the iceberg," said Michael Oppenheimer, a staff scientist with the Environmental Defense Fund. "It's a disgusting representation of the total problem . . . . One can envision a time when many of our coastal areas, our estuaries and bays, no longer present an acceptable habitat for living things."

From declining oyster harvests in Chesapeake Bay to closed shellfish beds in Texas to copper and mercury contamination in San Francisco Bay, the deterioration of America's waters has been a growing national problem. Near industrial harbors, more crabs, lobsters and flounder have been turning up with lesions and other signs of disease. "Dead seas," areas so depleted of oxygen that marine life cannot survive, are proliferating.

Despite these warning signs, it is the garbage that first washed up on Long Island July 6 -- eventually depriving millions of Northeast beachgoers of their summer refuge -- that may prove to be the watershed event that galvanizes public opinion about protecting the coastlines.

Nowhere is the impact more visible than on the New York beaches where swimming has been banned for weeks. On a recent, hot Sunday, Staten Island's Midland Beach was eerily quiet, its miles of sand undisturbed by footprints. Seagulls wandered aimlessly, searching for nonexistent scraps of food.

The scene was reminiscent of much of the Jersey Shore last summer, when a 50-mile garbage slick ruined the tourist season and triggered a legal war with New York City over whether the stuff came from the Staten Island landfill. But it wasn't until beaches were closed on this side of the Hudson River, where most of the national news media are headquartered, that the story was propelled onto the covers of Time and Newsweek.

Ironically, only a small portion of New Jersey's 127-mile coastline has had pollution problems this summer, yet tourism is down 30 to 50 percent because of the fallout over New York's beaches. "We're in pretty good shape, but nobody believes it, and the shore businesses are wilting on the boardwalk," said Jim Staples, a spokesman for New Jersey's environmental agency.

That is not to say New Jersey has been without problems. Four beaches in the Asbury Park area were closed when breakdowns at the city's now-defunct sewage treatment plant -- which earlier had been fined $91,000 for improper operations -- sent bacterial counts soaring. In another incident, 150 vials of blood, some contaminated with AIDS and hepatitis, washed up on the mud flats of Bayonne.

New Jersey environmental chief Richard T. Dewling said local officials should stop pointing fingers. "I was at Sandy Hook last week and I personally found 500 beer cans and one brassiere," he said. "We pick off our beaches 26,000 tons a year of garbage that people leave behind. We didn't have plastic tampon applicators on the beaches 15 years ago. We didn't have crack tubes on the beaches two years ago."

In Rhode Island, however, angry officials are considering suing New York and New Jersey over medical waste that has washed up at Newport and 30 other state beaches. Ronald L. DiOrio, Rhode Island's policy director, said some of the waste "has been traced back to the New York City area," including a bottle of hydrogen peroxide and a prescription with a name linked to a Brooklyn hospital.

Brendan Sexton, New York City's sanitation commissioner, called the charges "implausible" and an example of "New York-bashing."

Sexton's agency is among several here that have been investigating the waste that has repeatedly shut down beaches from Coney Island to Rockaway, some of which normally draw as many as 3 million visitors a month.

Late in the week, the medical waste problem moved south to Maryland. About 400 syringes, some with needles attached, were found washed ashore in Baltimore's outer harbor. State officials said the waste posed no imminent health threat and they insisted this problem was minor compared to New York and New Jersey.

In New York, one obvious, if unpublicized, factor is the city's combined sewer system, the underground network of pipes that carries both sewage and rainwater. During heavy rains, the system overflows, pouring raw sewage and fecal matter into area waterways through 400 outfall pipes. At least 10 percent of the system's sewage, or 60 billion gallons, is believed to overflow each year.

An antiquated sewer system was also the culprit in the temporary shutdown of nine Boston-area beaches at the end of July, when an estimated 400 million gallons of wastewater poured into Boston Harbor. State officials say such overflows occur 100 times a year.

Boston and New York also are two of the only remaining metropolitan areas that systematically deposit sewage in the ocean.

Each day, less than half a mile from the Boston shore, a Massachusetts sewer agency dumps 70 tons of sludge and nearly 500 million gallons of partly treated sewage. "It has coated the area within Boston Harbor with contaminated sediments and is probably a significant contributor to fish disease," said David Fierra, the EPA's director of water management in New England.

Meanwhile, New York City, two of its suburbs and six New Jersey communities unload nearly 2 billion gallons of sludge each year at a site 106 miles off Cape May, N.J. Cindy Zipf of Clean Ocean Action, a New Jersey environmental group, said this dumping continues because "it's cheap. The ocean is free and it's always open."

Until last winter, the sludge was dumped at a site just 12 miles off New Jersey. Zipf says commercial fishermen 70 miles away have complained that their nets have been coated with a substance she describes as "a foul-smelling heavy milkshake."

Although EPA officials say there is little chance of this sludge finding its way back to shore, the agency has sued to force an end to the dumping. Congress, which unsuccessfully sought to ban ocean dumping in 1981, is moving to end the practice by 1991. Last week, however, New York City officials pronounced that deadline unrealistic and said they cannot stop dumping before 1998.

Such battles have made water pollution the hottest political issue in New Jersey. In May, Democratic presidential nominee Michael S. Dukakis toured the New Jersey coast in a fishing vessel and pronounced the water "disgusting." Last month Vice President Bush went to Point Pleasant Beach and called for a ban on ocean dumping. New Jersey Gov. Thomas H. Kean (R) said last week he may impose a construction ban in coastal communities in an attempt to reduce pollution and some Massachusetts officials are pushing a similar ban on Cape Cod.

"The growth along the coastlines is incredible," said the EPA's Muszynski. "People want to live close to the shore, but not have the shore affected by the fact that they live there."

In New York, the already hot summer air has been filled with demands that those responsible for the beach closings be brought to justice. Mayor Edward I. Koch (D) recently called a news conference to announce that the manager of a Harlem medical office had been charged with dumping nearly 200 hypodermic needles, some dead rats and other waste into an outdoor trash can. But Koch acknowledged there was no connection with the beach closings.

Sanitation Commissioner Sexton, saying more arrests are imminent, argued that the blood vials and some of the syringes have been dumped by "venal" people.

"It's hard to imagine a way they could get there by accident," Sexton said. "The only imaginable scenario is a conscious dump by someone, either into a storm sewer or off the edge of the road. Whatever other problems the oceans have, we have the conscious act of one or more persons who wanted to be rid of this hot waste."

New York hospitals, which generate more than 40 million pounds a year of "red bag" waste, must pay stiff fees for contractors to haul the material to distant incinerators. Not surprisingly, many have cut corners. Some 50 city hospitals have been fined $481,000 since late 1985 for the sloppy disposal of infectious waste.

State environmental commissioner Thomas C. Jorling takes a less conspiratorial view of the problem. "There is no single source that can be identified as explaining the washups along the beaches," he said. "The evidence suggests multiple sources."

Most needles found on the beaches have been the kind discarded by addicts and diabetics, not hospital waste as originally thought, Jorling said. "There are over 60,000 people in New York City who use syringes on a daily basis," he said. "The basic way those materials are disposed of is in the trash, or down toilets, or in the sewer."

Some officials say the problems have been exaggerated by media hype. The 2,000 items of medical waste found on New York-area beaches, they say, could fit into six shopping bags. Initial sightings of intravenous tubes turned out to be popsicle containers, while laboratory rats on the shore were found to be sewer rats.

But learning the exact species of rodent is likely to be of small comfort to those who would like to hit the beaches without looking nervously underfoot. New Jersey's Dewling, for one, doesn't see the beach garbage as a great mystery.

"Where's it coming from? You've got 21 million people living in this area," he said. "That's where it's coming from. It's coming from you and me."