For almost two years, as the COVID pandemic shook life around the world, other infectious diseases were on the wane. Now that the world has rapidly dismantled measures to slow the spread of COVID, the viral and bacterial nuisances are returning – and behaving in unexpected ways.

Consider what we’ve seen lately.

The past two winters have had some of the mildest influenza seasons on record, but flu hospitalizations have spiked in recent weeks — in May! Adenovirus type 41, previously thought to cause fairly harmless bouts of gastrointestinal disease, can trigger severe hepatitis in healthy young children.

Respiratory syncytial virus, or RSV, a virus that usually causes illness in the winter, triggered major outbreaks of disease in children in the United States and Europe this past summer and early fall.

CLOCK: What is monkeypox and how does it spread as an outbreak escalates?

And now monkeypox, a virus generally only found in west and central Africa, is causing an unprecedented outbreak in more than a dozen countries in Europe, North America, the Middle East and Australia, with the UK alone reporting more than 70 cases of Tuesday.

These viruses are no different than before, but we are. For one, we have acquired immunity much less recently due to COVID restrictions; As a group, more of us are vulnerable right now. And that increase in vulnerability, experts suggest, means we may be getting a little… shaky as we work toward a new post-pandemic equilibrium with the bugs that infect us.

Major waves of disease could hit, which in some cases could bring to light issues that we didn’t know were triggering these bugs. Disease could circulate at times or in places where it would not normally circulate.

“I think we can expect some presentations to be exceptional,” said Petter Brodin, professor of pediatric immunology at Imperial College London. “Not necessarily really difficult. I mean it’s not a doomsday projection. But I think something out of the ordinary.”

Marion Koopmans, head of the Virosciences department at Erasmus Medical Center in Rotterdam, the Netherlands, said she believes we may be facing a time when it will be difficult to know what to expect from the diseases from which we thought we understood them.

“I think that’s possible,” said Koopmans.

This phenomenon, the disruption of normal infection patterns, could be particularly pronounced in diseases where children play an important role in spreading the bugs, she suggested.

Young children are usually germ magnets and germ boosters. But her life has been fundamentally changed during the pandemic. Most left at times without attending daycare or personal school. Many have had far less contact with people outside of their household, and if they encountered others, those people may have been wearing masks.

And babies born during the pandemic may have been born with low levels of antibodies passed from their mothers in the womb because those mothers may have been protected from RSV and other respiratory pathogens during their pregnancy, said Hubert Niesters, a professor of clinical virology and molecular diagnostics at the University Hospital in Groningen, the Netherlands.

Koopmans said a study her team conducted looking for antibodies in the blood of young children showed the effects of what she calls “infection honeymoon”.

“You can really see that children in the second year of the pandemic have far fewer antibodies to a range of common respiratory viruses. They just got less exposed,” she said.

Such factors may help explain the recent outbreak of unusual cases of hepatitis in young children. Scientists studying the cases believe they may be caused at least in part by adenovirus type 41, as it has been found in a significant number of the affected children. The possibility is puzzling as the virus has not been observed to cause this type of disease in the past.

However, some scientists believe that this virus has always been responsible for some of the small number of unexplained cases of pediatric hepatitis that occur each year. Perhaps, the reasoning goes, there have been many more adenovirus type 41 infections in the last eight months due to the increased susceptibility of children. This, in turn, could reveal something that had not previously been discovered.

“I think sometimes, to connect the dots of rare complications of common diseases, you just need enough cases out there to put the pieces together,” said Kevin Messacar, a pediatric infectious disease specialist at Children’s Hospital Colorado. “And there is some suspicion that this could be the case with the hepatitis cases.”

CONTINUE READING: A CDC expert answers questions about monkeypox

The disruption to normal mixing patterns caused by the pandemic means that even adults do not produce the levels of antibodies that would normally be acquired through regular exposure to insects, creating ever larger pools of susceptible humans.

For example, flu experts fear that if influenza viruses make a serious return, a gathering of people who have not recently been infected could result in a very bad flu season.

Koopmans said some studies suggest that after a period of a year or two when flu transmission is low, the number of people with flu antibodies high enough to be considered protective could drop significantly. “So possibly a larger, more vulnerable group in adults, too,” she said.

“We’re talking about endemic diseases that had a certain pattern of predictability. And that pattern was partly seasonal, but also partly driven by the size of the immunized or unimmunized population. And of course the last bit increased,” said Koopmans.

How will this work? All eyes will be on children’s hospitals this fall to see if there will be a spike in cases of a polio-like disease called acute flaccid myelitis, or AFM, which is thought to be caused by infection with enterovirus D68.

Messacar, who is also an associate professor at the University of Colorado, has been studying AFM for eight years, since the first of a series of biennial fall waves occurred in late summer and early fall in 2014, 2016, and 2018.

Then in 2020 nothing. The same in 2021. Does that mean there could be a much higher caseload in the fall of 2022 because more children are potentially susceptible to enterovirus D68? We have to be prepared for that possibility, Messacar said, stressing he doesn’t know what to expect.

“Now we have four years of children who have not seen this virus. We don’t know what will happen. We don’t know when it will come back. But if it does come back, there are more vulnerable kids out there who aren’t expected to have immunity,” he said. “We observe this with a large number of different viruses.”

Thomas Clark, deputy director of the Division of Viral Diseases at the Centers for Disease Control and Prevention, said people in public health have feared vaccine-preventable diseases could break out as many children around the world missed out on childhood immunizations during the pandemic .

But he said he now understands that the pandemic is not the only way to affect infectious diseases.

“We are very focused on under-vaccinated children with routine childhood immunizations as these are the prerequisites for measles introduction. But then there were also a lot of kids who didn’t catch the usual viruses that they might have been exposed to.”

Clark said we may see differences in the severity of some diseases because young children who were protected from bugs in the early stages of the pandemic may now get them when they are older. Some diseases cause more severe symptoms when transmitted in old age.

“Whether we’re going to see something like this in such a short amount of time is a big question mark, I think,” Koopmans said. “But I think it’s certainly something worth watching closely.”

Some experts believe a cluster of vulnerable people isn’t the only way the pandemic may have affected patterns of disease transmission.

CONTINUE READING: The “five pandemics” causing 1 million COVID deaths in the US

David Heymann, chair of a panel of experts advising the Health Emergencies Program at the World Health Organization, said lifting anti-pandemic measures could have helped encourage the spread of monkeypox in the current outbreak in Europe, North America and beyond. Many of the cases of monkeypox have been diagnosed in men who have had sex with men.

After two years of restricted travel, social distancing and public gatherings, people are shedding the shackles of COVID control measures and welcoming the return to pre-pandemic life. Media reports suggest that recent raves in Spain and Belgium have led to transmission of the virus among some attendees.

Heymann, professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine, reflected that the monkeypox outbreak in the UK or elsewhere outside Africa may have been simmering at low levels for some time, but may not have come to public attention until the international one travel picked up again.

“If you look at what has happened in the world in recent years and if you look at what is happening now, you might be wondering if this virus made its way into the UK two to three years ago and passed under the radar screen became. [with] slow chains of transmission,” said Heymann, who worked to eradicate smallpox early in his career. “And then suddenly everything opened up and people started to travel and mingle.”

While all of this could make for a worrying time over the next few years, things will eventually settle down, Brodin predicted.

“I think once you’ve infected a number of people, herd immunity sets in and the virus goes away,” he said, referring to viruses in general. “We haven’t fundamentally changed the rules on infectious diseases.”

This article is reproduced with permission from STAT. It was first released on May 25, 2022. The original story can be found here.