In the depths of the Covid-19 pandemic, something strange happened: For a year or two, illnesses that used to emerge like clockwork when fall and winter arrived — flu, RSV, and the myriad viruses that cause colds — did not sicken us.

The cause now appears clear: The measures we took to avoid the new disease, including isolating and social distancing, muscled most other respiratory pathogens out of the cold-and-flu-season picture.

When we started to tire of altering our lives to avoid SARS-CoV-2, which causes Covid, those other viruses returned — but at odd times and behaving in odd ways. Rhinoviruses — aka common cold viruses — made children in Hong Kong so sick they ended up in hospital. In late August and September of 2022 in parts of Europe and North America, RSV-infected children overwhelmed pediatric hospitals, months earlier than respiratory syncytial virus seasons occurred in pre-Covid times. Shortly thereafter, flu transmission kicked into gear, peaking just after Thanksgiving in this country — unusually early for influenza.

Now, as we head into the fourth Northern Hemisphere winter of the Age of Covid, respiratory viruses appear to be falling back into seasonal order, experts tell STAT.