Some patients develop symptoms of psychosis after COVID-19 infection.
It is possible that the stress of the pandemic is causing the psychiatric problems.
But some scientists think it could be the virus causing the body to attack the brain.
In 2020, a day after developing COVID-19 symptoms, a 30-year old man started thinking he could speak to his dead relatives.
The man, who had no history of mental illness, became convinced that the religious rapture was imminent, according to a case report published in August.
This psychotic episode went on for more than a month, during which the man knocked a door down, shoved his mother, and thought that he was being experimented on with radiation, according to the report.
He was given anti-psychotic drugs, but they had little effect. It is only after he was given medication usually used to treat autoimmune conditions that he got better, per the case study.
"Psychosis is one of medicine's big enigmas. We have a fairly poor understanding of what causes it and how it develops," Dr. Jonathan Rogers, a clinician and psychiatry researcher from University College London, told Insider.
Research suggests that psychiatric symptoms are common among COVID-19 survivors.
One study on the health records of more than 200,000 US COVID-19 patients found that about 13% received some kind of psychiatric or neurological diagnosis for the first time within six months of infection.
Psychosis - a particular psychiatric condition that is much more severe - affected only 0.42% of that group.
But such frequency was about twice that of people in the control group (patients who had the flu), according to the study.
This sort of increase could be for an indirect reason: the psychological stress that comes from having COVID-19, two scientists who spoke with Insider said.
But research suggests that something else could be going on: the virus could be causing the body to attack itself, making the brain malfunction.
The theory is that the virus causes so-called anti-NMDA-receptor encephalitis, an autoimmune reaction that causes brain inflammation. That in turn can cause psychosis.
Usually, the brain is protected from the immune system because of a structure called the blood-brain barrier.
But COVID-19 might make that barrier "leaky," said Dr. Benedict Michael, a clinician from Liverpool University.
"That then exposes immune cells to brain proteins that they wouldn't otherwise see," said Michael, who oversees a registry of neurological complications after COVID-19.
The immune system then starts attacking the cells in the brain, specifically, the NMDA receptors which are carried by neurons.
That in turn makes the neurons less sensitive to stimulation. "It's a similar effect to ketamine," he said, referring to the powerful sedative substance.
The scientists noted that another virus, called HSV-1, can cause similar brain problems.
The good news is that this kind of problem ought to be treatable with anti-inflammatory drugs and antipsychotics.
"We are hopeful that the majority will make a reasonable recovery because there's not been much brain damage," said Micheal.
But both Rogers and Michael said the theory should be taken with a grain of salt. There are only a small number of documented psychosis cases after COVID-19, and even fewer where antibody levels have been measured, they said.
The presence of the anti-NMDA-receptor antibodies could be unrelated to the psychosis, said Michael.
"It's possible that there's an immunological basis for these individual psych cases, but I don't think it's proven in terms of the treatment," he said.
"Psychiatry has a history of all kinds of treatments that are good if you give them to just one patient, but don't look so good when you do a clinical trial," he said.