(KERO) — For decades sea lions along the West Coast have been plagued by a lethal form of cancer. Now scientists have determined the cause of the disease.

The sea lions at San Francisco are a tourist favorite but their health has been in danger for a long time. In 1979 researchers discovered sea lions were dying from cancer caused by a sea lion-version of genital herpes. Since then up to one-quarter of all adult sea lions who have gotten it, have died.

"It's a sexually transmitted virus, which is actually a very efficient way to spread if you're a virus because you have to reproduce to keep the species going," said Dr. Alissa Deming the lead author of the cancer study.

It's believed that contaminated water is triggering the virus to create cancer. The sea lion cancer is similar to cervical cancer in humans so it may unlock some answers to preventing it in people.

There's is no treatment for the sea lions but researchers say they'd like to see something good come from their deaths.