In September 2011, 163 dead harbor seal pups mysteriously washed up on the shores of New England, and scientists have finally figured out why. The seafaring mammals had come into contact with a new strain of virus closely related to a kind of bird flu. But perhaps most worrisome about this newly dubbed "seal flu" is that it could eventually impact humans if it continues to evolve. Here, a concise guide to the flu strain:

How did the seals get it?
The flu in question evolved from H3N8, a strain that was first isolated in North American ducks in 2002. The pathogen is capable of making the leap from birds to mammals like horses and dogs, and now it infects young seals, too. (Most of the pups, which were either dead or dying when they washed ashore, were under 6 months old). Many animals, including seals, bats, pigs, birds, and whales, are susceptible to certain types of the flu that typically originate in birds and then spread to mammals. The swine flu, or H1N1, outbreak of 2009, for example, was a combination of bird, pig, and human-infecting virus bits coming together, says Wynne Parry at LiveScience. This new variety is considered a relative.

What does this new flu do?
The virus wreaks havoc on the immune system, and, in mammals, binds to cells in the respiratory tract, making it difficult to breathe. Usually it kills infected birds, and it left "horrifying skin lesions" on the seals, "a previously unknown symptom in flu deaths." What makes these findings especially concerning is that H3N8 mutated to infect a new kind of mammalian host in just a few short years. "Although birds carry a wide variety of flu viruses, which sometimes make the jump into mammals, they almost never acquire the ability to spread from mammal to mammal," says Alexandra Sifferlin at TIME. Researchers have determined that eventually this virus probably could.

But can it really infect humans?
While scientists still aren't certain if this virus could infect humans, the worry is that its alarmingly fast mutation rate may one day allow it to. When a "virus can adapt, evolve, and become more mammalian in phenotype," then "we really need to be concerned," says Dr. Anne Moscana, who edited the report on seal flu. Although the swine flu outbreak of 2009 killed just 0.002 percent of the millions who were infected, there's a possibility that this new variety could be much deadlier if it progresses. The fact is that "flu could emerge from anywhere, and our readiness has to be much better than we previously realized."

Sources: BBC News, Daily Mail, LiveScience, TIME