In recent months, strange photos of deer with tumor-like growths on their bodies have sparked fears of a new wildlife outbreak. Similar deformities had already been spotted on rabbits and squirrels, and now white-tailed deer across New York, Pennsylvania, and Wisconsin have been pictured with the same condition.

One Reddit user shared an image of a Pennsylvania deer, asking, “But what is the growth? Is it a mole? A boil? An injury?” Wildlife officials, however, say the cause is a well-known virus.

The condition is called deer cutaneous fibroma, or “deer warts,” and is caused by a virus transmitted between deer in all parts of the U.S. The growths, though alarming in appearance, are not new.

The virus spreads mainly through insects such as mosquitoes and ticks, which carry infected blood from one deer to another. It can also spread through direct contact with infected animals or contaminated surfaces. Belonging to the papillomavirus family, it is related to viruses that affect humans, though this strain does not infect people.

While humans cannot catch deer warts, the ticks that spread the virus can transmit diseases like Lyme disease, which has been appearing in new regions as temperatures shift. Experts warn that climate change is pushing illnesses into areas where they were never found before.

The warts usually appear on a deer’s neck, head, and forelegs. Though not painful, they can sometimes block vision, feeding, or walking. Typically, the growths dry up and fall off naturally after a few months.

One social media user described seeing the condition firsthand: “This deer in my yard… [the warts] got worse, then three months later, they were gone.”

Experts note that deer fibromas have been present in the U.S. since at least the 1950s. The recent wave of online photos has simply made more people aware of this long-standing virus.