H5N1 avian "bird" flu is making headlines this week, with new reports finding inactive virus detected in 1 in 5 U.S. milk samples.

That means the virus is infecting mammals such as dairy cows, and now researchers report it's turned up in a bottlenose dolphin in Florida.

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“We still don’t know where the dolphin got the virus and more research needs to be done,” said study co-author Richard Webby. He directs the World Health Organization's Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds at St. Jude’s.

Scientists are increasingly concerned that bird flu has moved from birds to numerous mammalian species, raising the chances that it might mutate to a strain that easily infects humans.

Right now, H5N1 is extremely rare in people, using coming only after prolonged contact with infected animals. But when people are infected with the virus, half of the time it is fatal.

That's why the emergence of H5N1 in yet another mammal sounds more alarm bells.

Reporting recently in the journal Communications Biology, Webby and colleagues say that the University of Florida (UF) Marine Animal Rescue team were alerted to a bottlenose dolphin in distress in Dixie County, Fla.

In a postmortem examination of the deceased dolphin, university scientists, along with experts at the Bronson Animal Disease Diagnostic Laboratory in Kissimmee, Fla., documented the presence of the avian flu virus in both the animal's lung and brain.

Other labs further confirmed the presence of the H5N1 virus. It's genome was compared to that of local birds, as well as viruses from Northeast seal populations. It's still unclear, however, where the dolphin contracted the infection.

“This investigation was an important step in understanding this virus” said study co-author Dr. Mike Walsh, an associate professor of aquatic animal health at UF.