When Long Beach lifeguards saw a thresher shark stranded on the beach on Sunday, it was still alive. At the direction of shark experts, they pushed it back out to sea and it swam around for a bit, but it just kept trying to come back ashore, according to officials.

By Monday morning, it was found dead on the sand near the Junipero Avenue lifeguard headquarters.

“If they make it back on the beach it’s a sign of a disease,” said Chris Lowe, director of Cal State Long Beach’s Shark Lab.

That disease is caused by carnobacterium, which is found naturally in the ocean, Lowe said. One of his graduate students recently found in a study that there are several strains of this bacterium, including one specific to sharks.

The bacterium gets into the brain of the sharks through the inner ear and starts eating the brain, eventually making the shark “like a zombie,” Lowe said.

“It sounds like a horror movie, right?” he said.

For years, salmon sharks have been washing up on shores, killed by the meningitis-like disease. Now scientists are finding other species of sharks, including mako and thresher, wash up with the same disease. Last year, dozens of salmon sharks beached themselves along the central and northern coast of California during a particularly large outbreak, according to Mark Okihiro, senior fish pathologist with the California Department of Fish and Wildlife.

Okihiro said he believes salmon sharks pick up the bacteria from feeding on salmon in Alaska and pass it on to their babies in utero.

“The hypothesis were working off of is that these sharks are born with the disease,” Okihiro said.

Most infected sharks are about 6 months to a year old when they die, he said. Okihiro has to wait until he examines the body of the Long Beach thresher shark to know its exact age, but Lowe told him its about 40 to 50 pounds, meaning it’s likely young.

Researchers don’t really know if this is a new phenomenon or if we’re just now paying attention.

“The problem is that we don’t have good records, say even 10 years ago,” Okihiro said. “These sharks could’ve been stranding and dying from the same bacterial infection, but if no one was seeing it, then we don’t know.”

Another problem with tracking the disease in sharks is that many times sick sharks will sink to the bottom of the ocean and researchers never come across their bodies, Lowe said.

Having a shark wash up on Long Beach shores is rare too, according to Marine Safety Chief Gonzalo Medina. His lifeguards were the first to come in contact with the shark and he said they noticed that it didn’t look so good even after they pushed it out into the water.

“It swam near shore, it didn’t make an exit out to deeper water,” Medina said.

For now, Lowe’s team at the CSULB Shark Lab has the shark’s body and will soon give it to Okihiro so he can perform a necropsy and determine what caused the shark’s demise.

Editor’s note: This article has been updated to correct the title and name of of Gonzalo Medina. He is the Marine Safety Chief.