The majority of the world’s population will experience some form of gradual hair loss or thinning over the courses of their lives. But when it doesn’t seem so gradual — and suddenly starts falling out excessively — it usually indicates something other than hereditary hair loss.

Some dermatologists say they’ve been hearing these exact concerns more frequently in recent years, and it’s no surprise why: Of the many causes of sudden hair loss, a high fever and stress are among some of the most common.

“There is no doubt that COVID presented this double-whammy to the system,” Dr. Mary Lupo, a board-certified dermatologist and an adjunct faculty member at Tulane University, told Nexstar.

COVID, Lupo said, resulted in patients experiencing not only physiological stresses on the body but also psychological stressors. Just one of these, however, could be enough to trigger delayed hair loss several months later.

At any given time, the hair on a healthy human head is going through one of multiple phases, including an anagen (or growth) phase, and a telogen (or resting) phase, with a much smaller percentage in the latter category. The telogen phase is relatively short, lasing only a few months before some of those hairs fall out.

But a high fever, or even a low-grade sustained fever, can result in telogen effluvium, a condition which triggers more follicles to enter the telogen phase and, in turn, causes excessive shedding at the end of that phase, according to the American Academy of Dermatology Association AAD.

Accordingly, acute telogen effluvium triggered by a high or prolonged low-grade fever occurs between “three and six months” afterward, Lupo explained. Studies published on the National Institutes of Health website further indicate that shedding could occur even sooner, with some subjects experiencing hair loss less than two months later.

The good news, Lupo said, is that this type of hair loss is usually temporary and grows back — but probably not as fast as any of the patients are hoping.

“It takes about a year for you to feel your hair’s back on track,” Lupo said.

It’s not only fever that can cause telogen effluvium. Events such as childbirth, hormonal changes, major surgery or discontinuing birth control medication can also cause shedding months later, according to the Cleveland Clinic and the AAD. But stress — and specifically stress over COVID-19 — was also likely a factor with folks who experienced telogen effluvium in recent years.

“Usually when you get the flu, you don’t typically worry about, ‘Am I gonna die?’ But COVID was different,” said Lupo, who added that some of her recent patients were experiencing more distress and even “hysteria” over their sudden hair loss than she had ever observed in her nearly four-decade career.

“I’ve never seen it be so upsetting,” she told Nexstar.

While there’s no way to completely stave off the negative effects that a high fever will have on human hair, Lupo did say that a healthy diet, and getting enough vitamin D, can help keep the scalp and follicles healthy. She recommends certain over-the-counter supplements that specifically target thinning hair, too.

It may also help to know that those suffering from sudden hair loss aren’t alone. Lupo estimated that around 50% — and perhaps as many as 80% — of people who experience a high or prolonged fever later begin shedding hair to some extent. It’s impossible to know for certain, she said, because some people may not seek treatment or even notice.

During the pandemic, however, many people did notice — and in droves.

“All of the pieces of the puzzle are there,” Lupo said of the COVID/stress combo the pandemic presented. “So it’s not surprising that we saw this.”