Ivan Agerton pulled his wife, Emily, into their bedroom closet. "I believe people are following me," he said.

He described the paranoid delusions haunting him: that people in cars were spying on him, that a SWAT officer was crouching in a bush in their yard.

It was a drastic change for Agerton, 49, a usually unflappable former Marine and risk-taking documentary photographer whose most recent adventure involved exploring the Red Sea for two months in a submarine. He was accustomed to stress and said that neither he nor his family had previously experienced mental health issues.

But in mid-December, after a mild case of COVID-19, he couldn't sleep, suspected ordinary people of sinister motives and eventually was hospitalized in a psychiatric ward twice. "Like a light switch — it happened this fast — this intense paranoia hit me," Ivan Agerton said. "It was really single-handedly the most terrifying thing I've ever experienced in my life."

His experience reflects a phenomenon that doctors are increasingly reporting: psychotic symptoms emerging weeks after coronavirus infection in some people with no previous mental illness. Doctors say such symptoms may be one manifestation of brain-related aftereffects of COVID-19. Along with more common issues like brain fog, memory loss and neurological problems, "new onset" psychosis may result from an immune response, vascular issues or inflammation from the disease process, experts hypothesize. While such extreme symptoms are likely to affect only a small proportion of COVID survivors, cases have emerged worldwide.

Much about the condition remains mysterious. Some patients feel urges to harm others or themselves. Others, like Agerton, have no violent impulses but become almost obsessively paranoid. Some need weeks of hospitalization while others improve faster. Some relapse.

Agerton spent about a week in a psychiatric ward in December. By mid-January, he seemed to have recovered. In February, however, "the paranoia came screaming back," Agerton said a day before being hospitalized a second time.

"I have all these questions," said Dr. Veronika Zantop, a psychiatrist who has treated Agerton. "Is this temporary? You know, how long does the risk continue?"

Paranoid delusions more commonly accompany schizophrenia in late adolescence or dementia in older adults, but so far, post-COVID psychosis has mostly afflicted patients in their 30s, 40s and 50s.

And some post-COVID patients have realized something was wrong, while typical psychosis patients often "don't have insight into their symptoms," Zantop said.

With Agerton, she said, "It's almost like he had a split self where he was able to say, 'My brain is telling me that the police are after me.' And then he was also able to say, 'I know that's not true on some level, but it feels like reality to me.' "

Agerton tested positive for the coronavirus in late November after returning from the Red Sea. With a low fever, mild respiratory symptoms and a loss of smell, he isolated in a bedroom at home on Bainbridge Island near Seattle for 10 days, protecting Emily Agerton, 46, and their children, ages 5, 11 and 16.

Then, on Dec. 17, an ordinary spam call triggered a cascade of paranoia linked to technology, surveillance and government agents. "I couldn't control myself," he said. "I was just thinking 'I'm losing my mind.' "

Recently, he said he felt considerably better, with 90% of the paranoia gone. It's unclear how long he will need medication and when he will be able to resume his adventurous work. "There's this fear of how long is this going to happen," he said. "How long am I going to live with this?"