For nearly two years, as the Covid pandemic disrupted life around the globe, other infectious diseases were in retreat. Now, as the world rapidly dismantles the measures put in place to slow spread of Covid, the viral and bacterial nuisances that were on hiatus are returning — and behaving in unexpected ways.

Consider what we’ve been seeing of late.

The past two winters were among the mildest influenza seasons on record, but flu hospitalizations have picked up in the last few weeks — in May! Adenovirus type 41, previously thought to cause fairly innocuous bouts of gastrointestinal illness, may be triggering severe hepatitis in healthy young children.

Respiratory syncytial virus, or RSV, a bug that normally causes disease in the winter, touched off large outbreaks of illness in kids last summer and in the early fall in the United States and Europe.

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 United Kingdom alone reporting more than 70 cases as of Tuesday.

These viruses are not different than they were before, but we are. For one thing, because of Covid restrictions, we have far less recently acquired immunity; as a group, more of us are vulnerable right now. And that increase in susceptibility, experts suggest, means we may experience some … wonkiness as we work toward a new post-pandemic equilibrium with the bugs that infect us.

Larger waves of illness could hit, which in some cases may bring to light problems we didn’t know these bugs triggered. Diseases could circulate at times or in places when they normally would not.

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

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

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

This phenomenon, the disruption of normal patterns of infections, may be particularly pronounced for diseases where children play an important role in the dissemination of the bugs, she suggested.

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