The transition from winter to spring signals the end of months of cold snaps, sweater weather, and flu season. But even as the roads clear and flowers bloom anew, it may not yet be time to shelve your sick days for the year.
According to a new article, published today in the journal PLoS Pathogens, all infectious diseases may be seasonal—and there’s at least one for every time of the year.
Study author Micaela Martinez, an infectious disease ecologist at Columbia University’s Mailman School of Public Health, presents a compendium of 69 infectious diseases that run the gamut from rare to common, neglected to notorious, innocuous to deadly. Despite their differences, all the syndromes have one thing in common: They rise and fall with changing seasons.
Martinez, a conservation ecologist by training, initially set out to track the seasonality of acute, or short-term, infections like influenza and chickenpox after noting similarities between the disease states of humans and wildlife. But as she began to compile a list of infectious diseases that tend to plague humans, Martinez found that the trend also held true for chronic, or long-term, diseases like gonorrhea and leprosy.
“There’s documented seasonality for all infectious diseases, which is not what I was expecting,” Martinez explains. “It’s an even more widespread phenomenon than we thought.”
A quick look at the infectious disease calendar paints quite the sobering picture. As autumn leaves turn from green to red, gonorrhea and yellow fever rear their ugly heads in some parts of the world. The winds of winter are famous for flu, but also bring bouts of pneumonia. Refreshing springtime blooms breathe new life into outbreaks of chickenpox and salmonella. And last but certainly not least, summer months pack the heat with spikes in Lyme disease, polio, syphilis, tetanus, tuberculosis, and more.
Other diseases are a bit less finicky: They’ll take any period of natural warmth, spreading the joy over many months at a time—often in regions of the world that don’t experience four seasons of climactic change. For instance, Chagas disease, diphtheria, and genital herpes all flourish over both spring and summer, while their prevalence takes a welcome dip when temperatures fall. Other infectious diseases fare better during rainy seasons, or when climes are particularly dry.
Though the diseases she linked together shared little in common, including even their seasons of prevalence, Martinez theorized there might be similar reasons driving their cyclic nature. To tease apart the connections, Martinez amassed data from 100 previously published studies, zeroing in on several factors that shape seasonality.
For instance, environmental factors obviously come into play. Climate conditions such as temperature, humidity, and rainfall impact the wellbeing of infectious microbes, as well as the humans and wildlife they plague. In diseases that are ferried to humans via an insect vector like a mosquito or fly, the seasonal ebb and flow of these pest populations may also play a role.
The behaviors of hosts of disease, both human and wildlife, also appear to fluctuate from month to month. This has been infamously exemplified by congregations of kids in schools begetting measles outbreaks. But there’s also something to be learned, Martinez says, by studying flux in the habits of non-human animals, who go through bouts of seasonal breeding, territoriality, and migration, and how these changes affect the spread of disease. Humans may not be seasonal maters—at least, not to the same degree—but even a subtle shift, like an uptick in sex during the summer months, could spur outbreaks of gonorrhea, genital herpes, or syphilis.
“It’s not that we are vulnerable at a particular time of year and healthy at another,” Martinez explains. “We’re restructuring throughout the year. And the identity of the thing we’re vulnerable to changes with the seasons.”
The idea that some infectious diseases are seasonal isn’t new, says Shanthi Kappagoda, an infectious disease physician and epidemiologist at Stanford University who was not involved in Martinez’ work. However, Kappagoda adds, Martinez’ framework is unique in that it includes some infections that haven’t traditionally been considered seasonal—including several sexually-transmitted infections—and may change how clinicians and researchers approach future epidemics.
According to Martinez, knowing when certain diseases are in season could help doctors quickly and effectively treat chronic infections that tend to flare up at certain times of the year, like herpes. With this sort of predictive power, patients may be able to minimize the amount of time they’re forced to deal with seasonal symptoms. The same line of thinking might even be applied to scheduling vaccines, which are ideally administered prior to the onset of an outbreak.
Understanding the drivers of infectious diseases’ seasonality may also be helpful on much broader scales, including the forecasting of epidemics worldwide, adds Amy Wesolowski, an infectious disease epidemiologist at the Johns Hopkins Bloomberg School of Public Health who did not participate in the new analysis.
Such global thinking is especially relevant in a rapidly changing world. As climate change progresses, the landscape of infectious diseases will shift, explains Kappagoda. Increasingly warmer locales may favor the transmission of summer-loving diseases like cholera. Humankind may also witness a boom in populations of insects like mosquitoes, which are likely to expand their habitats as temperatures climb, chauffeuring with them outbreaks of malaria, West Nile virus, and more. Additionally, human sensitivity to heat shouldn’t be underestimated in the context of infectious disease, Kappagoda points out: Climate change will continue to displace large populations of people, spurring the onset of epidemics.
Whether we like it or not, the spread and severity of infectious diseases are inextricably tied to both biology and behavior—which, in turn, cycle with the seasons. According to Martinez, the implications of this go far beyond fingering a malady for all seasons; rather, these patterns could change how we view our own bodies… and give new meaning to the phrase, “feeling under the weather.”
“This isn’t just about transmission—seasonality is also in the human body itself,” she explains. “There’s something happening in our bodies we don’t quite understand yet. Seasonality in infectious disease is just an enticing little piece of the puzzle.”