But why they are is a bit of a mystery. Researchers have speculated that changes in ambient temperature and humidity may make these viruses less viable by affecting their outer proteins and membrane, or by altering how quickly the virus-carrying droplets from mouth and throat evaporate. On the other hand, the change in infection rate might be due to the ways that people change their behavior with the seasons, going from being enclosed in offices and schools to opening windows and spending more time outdoors. (Getting more sunlight, which drives Vitamin D production and might enhance the immune system’s defenses, might play a role, too.)

More subtly, it’s also possible that changes in temperature and humidity affect our bodies in ways that make them more or less vulnerable to viral infection. Cold, dry air damages the cells that line the respiratory tract, which might leave them less able to recruit defenses against viruses. Warm, moist air might be protective: It hydrates the layer of mucus that lies on top of cells and insulates them from viruses and microscopic particles.

Whatever the reason, the seasonal effect for the flu and some kinds of colds is real: They peak during the colder months in both hemispheres. It’s different, though, in places that don’t have cold months, or seasons. In the tropics, flu infections occur year round. That also may have an environmental explanation. Humidity keeps virus-containing droplets from losing their water content; instead of becoming lighter, which lets them float through the air on the way to someone else’s nasal passages, they fall out of the air onto surfaces. And there’s some evidence that flu transmission in the tropics depends less on breathing the virus in and more on transferring it from a surface to a person’s eyes or nose or mouth. (In other words: Life finds a way.)

It turns out that the known human coronaviruses, the ones that are part of the mix of germs that cause common colds, have seasonality too. Last month, a team led by longtime flu researcher Arnold Monto of the University of Michigan analyzed 8 years of data from an ongoing study of respiratory infections that is normally used to assess whether the annual flu vaccine is working as intended. They found that four already-known coronaviruses—none of them the new pandemic strain—were circulating during those years, and that all four of them were what the team called “sharply seasonal.” They arrived in November or December, peaked in January or February, and disappeared after May. A study 10 years ago found a similar pattern in infections among kids in day care.

But those coronaviruses have been adapting to humans and interacting with our immune systems for a long time. Viruses that are new to humans seem not to follow that seasonal rule. Take, for instance, the 2009 H1N1 flu. It wasn’t a seasonal strain when it debuted; it was a new-to-us virus that crossed over from pigs and began infecting humans as the regular flu season was ending. It continued to infect people throughout the summer, and then the case count rose into the fall and winter. By the following year, though, it had become another seasonal flu strain.

“We have seen before that novel pathogens behave differently in their first year than after they become established and have an opportunity to circulate in the human population,” says Caitlin Rivers, an epidemiologist and assistant professor at the Johns Hopkins Center for Health Security.

Making matters more complicated, the coronavirus that broke out in 2003, causing the worldwide epidemic of SARS, moved through the world in February but was completely gone by June. Some of its squelching was due to aggressive interventions, such as quarantine and social distancing. But some may have been due to rising temperatures. A study published in 2011 by researchers in Hong Kong, where the SARS outbreak was intense, found that the virus did survive better in cool, dry places. They hypothesized that might have protected some parts of Southeast Asia, but might actually have facilitated transmission in Hong Kong and Singapore, because those affluent cities have so much air conditioning.

Read all of our coronavirus coverage here.

And, to be clear, that coronavirus is not this one. The virus behind Covid-19 is perplexing in so many ways, from the people it attacks to the range of illness it causes to its wild array of symptoms. Unlike its cousin, the original SARS virus, it seems perfectly happy to spread in tropical climates. And new research, also reviewed by the National Academies committee and subsequently published as a preprint, shows it may be extremely sturdy even in conditions that would daunt other viruses.

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