Strange photos of deer with tumor-like growths have sparked fears of a wildlife outbreak. A Reddit user asked, “But what is the growth? Is it a mole? A boil? An injury?” Officials say the cause is a known virus.

The condition, called deer cutaneous fibroma or “deer warts,” is caused by a virus spread mainly through mosquitoes, ticks, and direct contact. Though related to human papillomaviruses, this strain does not infect people.

Humans are safe from the warts, but ticks spreading the virus can carry diseases like Lyme. Experts note that climate change is pushing illnesses into new regions.

Warts usually appear on a deer’s neck, head, and forelegs. They are not painful but can block vision, feeding, or walking. As one observer said, “This deer in my yard… [the warts] got worse, then three months later, they were gone.”

Deer fibromas have existed in the U.S. since the 1950s; recent online photos just raise awareness of this long-standing virus.