There’s nothing worse than getting sick in the summer, and no group knows that better than the Island seal population which hopes to shake off a mysterious illness that has brought several weary seals ashore to die over the past couple months.

The native seal population has grown steadily since the passage of the 1972 Marine Mammal Protection Act, and the once-rare sight on Vineyard beaches of their sleek obsidian heads poking out of the surf has become commonplace. The federally-protected pinnipeds were once considered a nuisance to fishermen but now their resurgence has welcomed a burgeoning seal tour industry, and perhaps less welcome to intact beachgoers, the attendant return of their apex predator, the great white shark. It is unknown whether the higher seal population density has contributed to the emergence of this latest malady, but Chris Kennedy, Vineyard superintendent for The Trustees of the Reservations, described this year’s haul of sick seals as “really unusual.”

“We don’t usually find this many ill juvenile seals,” he said on Thursday. “There have been quite a few harp seals and some gray seals. There were a couple of adults but of the 15 to 16 that came ashore about a dozen or so were juveniles.”

Mr. Kennedy said the seals had cloudy eyes accompanied by a pus-like discharge. That and an uncharacteristic lethargy in the animals was a clue that they were unwell.

“It’s indicative that something is wrong with animals but I have no idea whether it was respiratory illness or a parasite or what,” he said.

Convalescing on Norton Point, Cape Pogue and what is left of badly eroded Wasque, many of the ailing pups did not pull through.

Mr. Kennedy said the standard procedure is to put in a call to the New England Aquarium which dispatches a local member of their marine mammal stranding network, in this case Sarah Trudel, also of the Trustees, who typically will send pictures and descriptions of the afflicted seals back to aquarium experts.

On Monday New England Aquarium marine animal rescue senior biologist Adam Kennedy said his team had not heard about the recent plight of the Vineyard seals, but said that it is not unusual to see respiratory illnesses such as pneumonia move through a population in the springtime. Other seals may be the victims of their own strange habits.

“Harp seals are sometimes known to ingest rocks and we’re not sure why they do that,” he said. Mr. Kennedy surmised that the animals, which are weaned in the Arctic, may be dehydrated and accustomed to chewing on pack ice for relief. Other animals are simply exhausted and Mr. Kennedy’s schedule has been hectic of late with calls from Nantucket to New Hampshire reporting the yearly crop of beached and disoriented gray and harbor seal pups.

“Most calls we receive are what we call weaner seals,” he said. “They’ve been weaned by mom and just they’re still pups trying to figure out life on their own. Some of them do really well for themselves and you know some of them, well, it’s natural selection. We do lose a certain percentage every year. Failure-to-thrive animals, we call them.”

Mr. Kennedy said he awaits more information about the Vineyard seals to determine the cause of this year’s die-off, but he said by all accounts it has been a strange year for some of the local marine mammals, such as an unusually peripatetic group of harp seals.

“We typically see them north of Cape Cod in the winter but this year we had reports from as far south as North Carolina,” he said.

Presumably all of the harp seals, so called for their telltale dorsal wishbone pattern, have returned to the Arctic for the summer, but of this past winter’s northern visitors, Mr. Kennedy said adults far outnumbered juveniles, an altogether unexplained phenomenon. Typically juvenile harp seals wander farther south than adults, content to stray far from home to forage rather than deal with the volatile behavior of adults during breeding season.

“It was a crazy year for harp seals,” he said.

At Trustees properties, Mr. Kennedy warned concerned beachgoers that a hands-off approach is best when encountering seals on the beach and that simply because a seal is alone does not mean that it is distressed.

“Often nothing is wrong with the animals,” he said. “They’ve just been kicked out by their mothers. Usually they come on shore just because they’re tired.”

He said well-meaning passersby can sometimes make life more stressful for the animals by investigating what they believe to be a sick seal.

“What’s disturbing is when we’ll see a young seal pup which has come up on the shore simply to rest and we’ll come back a couple of hours later and see a fresh set of footprints walking up to it and the seal will go right back in water,” he said. “It just wanted to take a break but it couldn’t.”