It may be hard to imagine in an artificial, light-filled and constantly stimulated environment, but humans are supposed to buckle down and rest when winter months arrive. Although we have acclimated to colder environments through behavior adaptations over time, humans began in a tropical climate and are not naturally well-adapted to the cold. To survive, we learned to build shelter and preserve food, activities that, while life-saving, may also expose us to new microbial threats and challenges. 

Respiratory Infections: Why Do They Happen More Often in Winter? 

The influence of cold weather on the likelihood of contracting respiratory infections has been noted over hundreds of years. Influenza is derived from the Italian term influenza di freddo, which means "influence of the cold," and was used in Italy during the 1300s to describe the illness. While most have probably heard at least once in their lives that it's dangerous to go outside with wet hair because one will catch a cold, that is, in fact, a myth. As it turns out, lack of humidity is a significant driver of influenza infections in the winter. Influenza is spread by respiratory droplets in the air, which in warmer, humid weather take on more moisture and fall to the ground faster. In cold, dry climates, the respiratory droplets can stay in the air for longer, thereby infecting people more often. Combine this with indoor crowding and the closeness that comes with moving indoors and trying to stay warm, and it is a recipe for respiratory ruination.

Food Storage 

One of the most significant challenges associated with cold winter months is a lack of fresh food, and human survival has long depended on storing enough summer and autumn food to last until spring. Research demonstrates that ancient hunter-gatherers used the help of bacteria living in lakes to preserve meat during the winter months. It is believed that after successfully hunting a mammoth, indigenous people placed the meat in small, cold lakes where lactobacillus bacteria produced lactic acid and preserved muscle mass. Lactic acid-producing bacteria have long been used in the food industry to prevent food spoilage, since lactic acid prevents bacterial growth and the production of biologic amines, which ultimately contribute to the breakdown and rotting of food. Simply stated, ancient hunter-gatherers figured out how to extend the “shelf-life” of the food they hunted.

Though bacteria may play additional roles in preserving food, through pickling and fermentation, they can also lead to deadly food spoilage. In 1795, Napoleon put out a call and offered a large reward for anyone willing and able to invent a new method of food preservation. The popularity of canning picked up in the U.S. after the tin can and the mason jar were developed in the 1800s. By World War 1, canning was a major part of the war effort; however, this new methodology was not without risks. Despite development and publication of the U.S. Department of Agriculture (USDA) recommendations for proper canning practices in 1917, several people died from improperly canned peas contaminated with Clostridium botulinum in 1931. This was followed by the largest botulism outbreak recorded in U.S. history, which resulted from improperly canned peppers served at a restaurant in 1977.

In Canada, botulism outbreaks have been centered around the country’s native population since 1985. Most of the reported cases since 1985 have been due to the contamination of traditionally prepared marine mammal meat, particularly from seals and Beluga whales.

Clostridium botulinum is an anaerobic Gram-positive bacillus that produces one of the most potent known neurotoxins, which can cause muscle paralysis and death if ingested. Therefore, proper canning precautions must be taken to prevent contamination and subsequent illness when prepping food for a long winter (or quarantine).

The Little Ice Age and the Plague 

From the early 1300s to 1850, the world experienced a significant cooling period called the "Little Ice Age." During this time, average global temperatures dropped nearly 4°F, which was enough to significantly impact crops and cultural practices worldwide. A massive grain shortage in Europe and North Africa necessitated import of grain from the Middle East. However, when the first trade ships arrived, most sailors were already dead, and survivors suffered from advanced signs of bubonic plague, including oozing buboes and pneumonia.

The cause of such destruction onboard the trade ship was an infestation of rats, which carried fleas harboring the Yersinia pestis bacteria. These fleas found new hosts in their European and American destinations, ultimately infecting new rats. The rat population may have exploded, in part, due to increased biomass resulting from increased precipitation during the Little Ice Age. At the same time, the cool and humid conditions created by the Little Ice Ace may have supported flea survival and breeding. Combined with poor sanitary conditions and crowding, the plague killed over 20 million people in Europe by the end of the epidemic.

While the incidence of plague did not seem to increase during the winter months, the Little Ice Age serves as a great example of how cooler temperatures and climate change can significantly impact agriculture and cultural practices, which in turn may introduce pathogens into populations that have not previously experienced their wrath.

Make Winter Safer 

The chill of the winter season encourages people to slow down, retreat and reflect. The increased hours of darkness and quiet allow one to think a little harder about staying healthy and protecting others during the winter season. To prevent the spread of respiratory viruses, cover your cough or wear a mask. Maintain distance from others if you are sick, and avoid attending or hosting crowded events. Ensure that all food is cooked and prepared correctly to prevent foodborne illnesses. Finally, consider steps that can be taken to help protect the planet and reduce the impact of climate change; we never know which pathogen is around the corner waiting for the opportunity (and ideal conditions) to strike.