A Colorado man was one of two unlucky people who developed nerve damage after a monkeypox infection, and federal officials urged doctors to watch for the rare but serious complication.

The Centers for Disease Control and Prevention published a report Tuesday describing the cases of two previously healthy men who had to be hospitalized because of nervous system complications from monkeypox. Almost 22,000 people nationwide have tested positive for monkeypox, including 271 in Colorado.

Most people with monkeypox develop a rash, which is often painful, and some have flu-like symptoms. Some develop only one or two lesions, and may not realize they have the virus.

Serious complications from monkeypox are rare. One person was confirmed to have died from monkeypox in California, and a person being treated for the virus died in Texas, though it’s not yet clear if that was the cause of death.

The Colorado patient was a man in his 30s who identified as gay and had no known health conditions before his infection. About nine days after he first noticed flu-like symptoms, he developed unexplained weakness and numbness in his legs and his left arm, and was hospitalized.

An MRI found spots in his brain and spinal cord where something had attacked the protective covering on the nerves. It’s not yet clear if the virus directly invaded the patient’s nervous system, or if an immune overreaction was responsible for the damage.

While the Colorado patient’s weakness improved, he still had to go to a rehabilitation facility and was using an assistive device to walk one month later. (An assistive device could be something like a walker or cane, though the CDC’s report didn’t specify.)

The other patient, a man in his 30s who lived in Washington, D.C., had an even rougher time. He had to be placed on a ventilator, though he eventually recovered enough to go to a rehabilitation facility.

Both patients received multiple drugs, including Tpoxx, an antiviral that was developed to treat smallpox but is being tried against monkeypox. Both improved after a treatment that filtered their blood plasma, though that could be coincidental.

The CDC urged doctors caring for patients with monkeypox to track and report complications from the virus. The current outbreak was identified less than six months ago, so there isn’t much data about how rare neurological complications are, or how best to treat to them.

Subscribe to bi-weekly newsletter to get health news sent straight to your inbox.