It’s a common complaint this winter: After coming down with a respiratory illness, some people feel like they can’t shake a lingering cough or runny nose despite other symptoms going away. Or they start to recover then see symptoms return a week or two later.

Doctors say that course of events isn’t unusual, though it might be more pronounced this year.

Covid, flu and respiratory syncytial virus (RSV) are all circulating widely. As of Dec. 16, flu hospitalizations had increased nearly 200% over the previous four weeks. And Covid hospitalizations increased around 40% over the four-week period ending Dec. 9, the latest data available.

NBC News spoke with seven doctors across seven states about why some people’s symptoms can persist for weeks or months. They offered several possible explanations.

For one, the experts said, many people are more susceptible to respiratory illnesses this winter because they haven’t had a recent infection or vaccination. Others may have gotten back-to-back infections that they confused with lingering symptoms.

It’s also likely that, following the pandemic — when many common viruses weren’t circulating widely — some people simply forgot how long symptoms can linger after a standard respiratory illness.

“It can take as long as two weeks or more to recover fully,” said Dr. Linda Bell, South Carolina’s state epidemiologist.

‘Immunity debt’ may be catching up to people

Because masking and isolation slowed the spread of many viruses during the pandemic, some people haven’t been as exposed to flu or RSV over the last few years as they would have been. That can give rise to what doctors call “immunity debt” — decreased immunity that makes people more susceptible to infection.

“As more of us are encountering these viruses that we haven’t seen in recent years, it might feel like some of them are a little bit more severe and we have some more severe symptoms than we had before,” said Dr. Molly Fleece, a hospital epidemiologist at the University of Alabama at Birmingham Medicine.

A lack of vaccine-induced protection can also predispose people to more severe illness and make it harder to recover, doctors said.

The Centers for Disease Control and Prevention sent an alert to health care providers last week warning about low vaccination rates for Covid, flu and RSV.

RSV vaccines have been approved for older adults and pregnant people, but just 17% of those ages 60 and older had gotten an RSV shot as of Dec. 9. Meanwhile, the CDC reported a supply shortage for a newly approved RSV antibody injection for infants in October. However, additional doses became available last month, and 230,000 more are expected in January.

The flu vaccination rate so far this year is 42% for adults and 43% for children, compared with 47% and 57%, respectively, the previous season. Just 18% of adults and 8% of eligible children have received the newest Covid vaccine.

Last year, doctors said, masking and social distancing were also still more common.

“That may be why people are getting more sick now,” said Dr. Caroline Goldzweig, chief medical officer of the Cedars-Sinai Medical Foundation in Los Angeles.

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Two infections back to back

Since this is only the second year with Covid, flu and RSV circulating widely at once, there may simply be more opportunities to get sick than in previous winters, doctors said.

“In the past several years, we’ve had primarily Covid or primarily RSV peaking, and now we have multiple respiratory viruses all rising at the same time,” said Dr. Larissa Pisney, an infectious disease specialist at UCHealth in Aurora, Colorado.

That could raise the chance of back-to-back infections.

“It’s entirely possible to be exposed to several different viruses over the course of the winter and have several different bouts of respiratory infection,” said Dr. Daniel Ouellette, a pulmonary disease specialist at Henry Ford Health in Detroit.

It’s also possible to get more than one virus at a time, though the CDC hasn’t noticed that happening much, according to its director, Dr. Mandy Cohen.

“We see co-infections at about a similar level to this time last season,” Cohen said.

However, several doctors said they’re seeing an increase in bacterial infections — such as strep throat, whooping cough or pneumonia — that either follow a viral illness or occur at the same time.

“In some situations, having a viral respiratory illness increases your risk of having bacterial pneumonia, which we’ve classically seen for a long time with the flu, but then we also saw a little bit of that with Covid as well,” said Dr. Shivanjali Shankaran, an infectious disease physician at Rush University Medical Group in Chicago.

It’s normal for symptoms to linger or recur

Symptoms that go away then reappear could be part of the same viral infection, said Dr. Donald Yealy, chief medical officer at the University of Pittsburgh Medical Center.

“You can have an initial infection, start to get better and then have some recrudescence — in other words, recurrence of the symptoms as you’re recovering,” he said. “People may mistake that for two separate infections.”

It’s also fairly common to feel sick for several weeks, doctors said. And Covid, flu and RSV can all result in a post-viral cough.

“That post-viral cough doesn’t necessarily mean the person is still potentially able to spread the infection to others. It’s just a residual effect of their prior infection,” Fleece said.

But a small minority of people may not recover for months or years. Long Covid affects around 6% of U.S. adults, according to a June survey by the Census Bureau. Similarly, it’s possible to see lingering effects from flu or colds.

A study published last week showed that the flu can lead to a persistent cough or shortness of breath over the course of at least 18 months. And an analysis of U.K. adults published in October found that common cold viruses can lead to coughing, stomach pain and diarrhea more than a month after an initial infection. Scientists are still trying to understand why.