We thought we were safe. The flu season was tapering off. Spring was in the air (sort of, some places). As flu cases flatlined through January and February, Americans breathed a sigh of relief, kicked up our collective feet … and immediately realized we’d accidentally ingested norovirus and made a break for the restroom. 

Welcome to the new season of disease. The good news is that symptoms of norovirus don’t last very long — though you can keep shedding virus for a long time afterwards so please wash those hands. The bad news is that this is just going to keep happening. We’ll never be out of “disease season,” only the disease will change. While the public — and even the infectious disease community — often talks about the seasonality of disease as something that only applies to the flu in winter, the evidence shows that every single disease has some element of seasonality to it, said Micaela Martinez, director of environmental health at the nonprofit We Act for Environmental Justice.

But while all infectious diseases have a seasonal component, the factors creating seasonality aren’t the same for each. Take influenza and norovirus. Both tend to be winter diseases in the Northern Hemisphere, coming around again and again with the cold. But these viruses spread in really different ways. Flu is a respiratory virus, spread by breathing and coughing. Norovirus is ingested — something you’re particularly likely to pick up from food prepared or touched by people who haven’t washed their hands thoroughly after using the toilet. 

Flu is a winter virus at least partially because of the way temperature and humidity in colder months combine to help it survive better outside the body, Martinez said. But norovirus is already a very tough critter. Instead, its tendency to peak in late winter — February or March — may be tied to a combination of wet conditions that increase flood risk (and potentially expose people to tainted water) and an increase in travel, especially to places like cruise ships, where lots of people are touching food, plates and silverware at buffet meals. 

The seasonality of a disease can be affected by the weather, the behavior of vectors that carry it or human behavior itself. Sometimes it’s a combination of all three. Scientists don’t even always know exactly why a specific disease has a specific seasonal pattern in one place or another. Flu has probably one of the best-documented seasonal patterns of infection, and scientists still debate exactly what combination of factors causes it. What is clear is that seasonality and the probable causes behind it will differ — even for the same disease — depending on things like geography, local culture and the development of medical technology.

Measles and chickenpox, for example, used to spread rampantly among children in the U.S., with yearly peaks happening when kids were in school, crammed together in their classroom petri dishes. 

Cases tended to drop off in summer, when the children dispersed. In some other countries, though, patterns of infection for those diseases have been tied to agriculture, Martinez said, with peaks that coincide with rural farmers congregating in population centers to sell their harvests. Meanwhile, successful vaccination campaigns have meant that, even if measles and chickenpox do still crop up occasionally in this country, we no longer expect outbreaks every spring. 

Seasonality of disease can also be affected by human behaviors that you didn’t even realize were seasonal. “Very strong data shows that there is very strong seasonality in sexual activity around the world,” Martinez said. “And essentially, people have more sex on weekends, more sex during summer holidays and more sex during actual holidays like Christmas, New Year's, Valentine's.” The result of this, she told me, is that sexually transmitted diseases — such as gonorrhea — are also somewhat seasonal. Summer is gonorrhea season. 

And because seasons don’t happen at the same time everywhere, disease seasonality doesn’t either. Take Lyme Disease, the peaks of which are linked to the highest level of activity in its animal vector: The nymph stage of the black-legged tick. That can happen as early as spring in hotter parts of the country, while the peak for New York state doesn’t happen until late July or August. 

All of this means that climate change is likely to have big impacts on infectious diseases, and that those impacts are going to vary widely from place to place and disease to disease, Martinez said. The shifting seasonality of disease will mean something different in a place that gets more heavy rainfall (gastrointestinal diseases like norovirus love flood conditions) and earlier, hotter springs (great for the spread of certain species of mosquito) than it does in a place that’s experiencing historic drought.

But the fact that we know diseases are seasonal could give us some powerful tools to protect people, Martinez told me. Back in 2015, during the Zika virus outbreak in South and Central America, women were being cautioned against getting pregnant, Martinez said, because of the way infection can harm fetuses, causing more babies to be born with microcephaly and developmental delays. But there’s still no vaccination for Zika, and people are still going to want to have sex and babies. This is the kind of situation where Martinez sees solutions in seasonality. If you know the seasonal patterns of Zika, she said, you could use that to plan pregnancies — using various methods of birth control to reduce the risk of having the most vulnerable time of a pregnancy coincide with the highest Zika risk. 

“I think we need to get past the point of just documenting seasonality,” Martinez said. “Can we use the seasonality of infectious diseases to actually get better about how we intervene? It might open the door for opportunities.”