Strep. Norovirus. COVID-19. Adenoviruses, enteroviruses, nondescript respiratory viruses — a choose-your-own-adventure of days-long misery and malaise. Then, two weeks later ... again?

Many people say they’ve suffered back-to-back common illnesses in recent months that seem both more frequent and more severe than what is typical for springtime. Although data on common infections is not collected on a large scale outside of the winter respiratory virus season, the Centers for Disease Control and Prevention reported a recent surge in strep throat — even among adults, who are usually less susceptible — and outbreaks of norovirus, sometimes called the “stomach flu.”

The timing of the onslaught — in the wake of a three-year, once-in-a-generation pandemic followed by a winter virus season for the record books — makes it all the more painful to endure. People are left wondering what could be going on, and when will we all catch a break?

The answer is complicated, physicians and public health experts said, and there’s no single factor to blame. A combination of deer-in-the-headlights immune systems, caught off guard after years of vigilant protection; a decline in many facets of overall health post-pandemic, and the shifting of illness to the forefront of our collective and individual psyches all come into play, they said.

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The early, severe onset of RSV and influenza starting in late summer last year provided the first clue that immune systems, so well-guarded by masks and distancing during the pandemic — especially in states like Maryland, with robust public health measures and a citizenry committed to following them — were more vulnerable to seasonal illnesses after years of little exposure. People had begun mingling again while largely eschewing masks, leading to unchecked circulation of viruses, especially in those who had not received available vaccines.

Current upticks in illness can still be partly attributed to immune systems’ lingering lack of experience fighting off infections that, under normal, pre-pandemic circumstances, may routinely circulate this time of year, said Dr. Gregory Schrank, infectious disease specialist at the University of Maryland Medical Center and assistant professor at the university’s School of Medicine.

“Our immune systems don’t have the recent memory that would help prevent infection again after exposure,” Schrank said, and this has led to “very high levels” of different infections spreading intermittently and rapidly though schools and workplaces, “and we’re seeing a lot of people sick all at the same time.”

Hibernating, shape-shifting antibodies

When exposed to an illness, immune systems release antibodies, a type of proteins produced by the blood, which attack viruses or bacteria.

Lack of recent exposure means fewer antibodies. Once generated by an initial infection or a vaccine, antibodies will wane over time, but never disappear completely, Schrank says. An adult who has had strep in their lifetime still harbors antibodies, though less of them, which makes the response less rapid and strong. So, people will typically get sick before the immune system kicks in and mounts its defense, he said.

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The body’s more sluggish response makes people feel like they’re getting sicker from common illnesses than they used to, said Dr. Niharika Khanna, primary care physician at the University of Maryland Medical Center and professor of family medicine at the university’s School of Medicine. “That’s kind of upsetting a lot of people,” she said, especially when they’re getting sick more often as well.

Population immunity also affects spread of disease, Schrank said, just as it did during COVID. Without “running up against the wall” of someone with a high level of immunity who is able to disrupt the chain of transmission, illness-causing germs will continue to circulate unimpeded, he said.

“Those people who would have carried the antibody for, say, two months and not caught the next cold virus, probably now get sick,” said Khanna.

Some antibodies decay more rapidly than others. Antibodies produced in response to the viruses causing measles or mumps — whether from a vaccine or natural illness — stick around in quantities which, except in rare cases, remain large enough to confer lifetime immunity. Common respiratory infections, though, don’t elicit as strong of an immune response, so antibodies are produced in lesser quantities that wane much quicker — typically in a matter of weeks or months.

A virus’s tendency to mutate, and the rate at which it does so, also comes into play. The flu virus, for example, mutates fast enough to necessitate annual revisions to the vaccine, and other respiratory and gastrointestinal viruses can also mutate rapidly, which either negates or lessens the effectiveness of antibodies produced against similar strains. Polio and measles, on the other hand, are highly unlikely to mutate enough to evade immunity, whether it’s gained through an infection or a vaccine.

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Still, reduced opportunities for immune systems to “exercise” during the pandemic does not mean they were damaged or rendered less effective by masking and other public health precautions, Schrank said. In fact, we can be certain they respond as well as they ever did, he said, given how effective COVID infection and vaccination proved to be in preventing severe disease from later exposures.

‘A heightened state of alert’

People’s perception of illness has also been transformed by the pandemic, said Keri Althoff, epidemiologist and associate professor at the Johns Hopkins Bloomberg School of Public Health. This impacts how they react to getting sick or to the spread of illness in their communities, and the amount of mental space this occupies.

Althoff said that people are more sensitized to “detecting these illnesses in our bodies” now, since “we have been at such a heightened state of alert” monitoring for symptoms of COVID and other infections for the past few years.

Schrank said the increased awareness of infectious syndromes leads not only to a more keenly felt experience of illness, but also carves out a larger place for it in mainstream discourse, as people have become more likely to discuss their experiences with others, speculating on their symptoms and what they could mean.

Khanna said in the past, when her patients had a cold, “they would probably not call me.” Now, though, they probably will, she said.

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Patients are usually concerned they might have a more dangerous virus, like COVID or flu, but even when they test negative, they’re still worried they could have something bad, she said.

They want to know, “Am I going to be OK?” Khanna said. “That’s the big question.”

An overall decline in health during the pandemic may also be partly to blame for increased illnesses now, Althoff said. In general, people consumed more alcohol, engaged in less physical activity, and may have foregone needed care for chronic diseases. Mental health worsened during the pandemic and so far has not improved.

It’s important for people to take stock and “think holistically about their health,” Althoff said. This includes fundamentals like sleep, diet, exercise, and basic hygiene practices like hand-washing, she said, which will not only help protect people protect against circulating bugs, but will also help them become healthier generally.

A light at the end of the tunnel?

It’s hard to know when things will “normalize,” Althoff said, with regard to the immune system’s susceptibility to common illnesses. This stems in part from the rarity of pandemics and other events that would produce similar conditions of isolation, limited exposure, and resulting vulnerability to infections that could then be studied to find out when preexisting levels of protection are regained.

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If anything, the forthcoming respiratory virus season is expected to follow a more normal pattern, Khanna said, with onset of flu and RSV occurring closer to the usual timeframe. This will allow the health care system to be more organized and prepared, she said.

But it’s likely that higher rates of disease will continue to “impact people’s daily lives” over the course of this year or the next few months, Althoff said.

This article has been updated to correct the spelling of Dr. Gregory Schrank's name.

Sarah True was a public health reporter for the Baltimore Banner. She previously worked as a freelance journalist covering healthcare and health policy, and has been both a medical social worker and a health policy analyst in a past life. She holds dual Master’s degrees in public health and social work.

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