"If you don't cover yourself, you'll get sick". Many of us have heard this phrase before. Cold temperatures, and the seasons associated with them, have long been associated in society with various infections such as colds or the flu. Indeed, several studies have shown that the propensity of sick people was generally higher during cold periods. So much so that science ended up looking into the matter and trying to identify the causes of this higher frequency of infections in winter and autumn. So why do we get sick more often when it's cold?
The common cold is caused by a number of pathogens, viruses, which are transmitted from person to person in the form of droplets from the respiratory system. The culprits are actually hundreds of strains of rhinoviruses and coronaviruses. These viruses are most active during the winter and fall months, and they need human hosts to survive and thrive. In the same way, the flu is also caused by viruses of the influenza family. Since these months are at the beginning and end of our coolest seasons, we have historically associated cold weather with illnesses caused by these viruses. But how responsible is the cold?
Some research suggests that rhinoviruses can replicate more efficiently at temperatures below 37°C, which is the average human body temperature. The temperature inside the nasal cavity is around 33°C, which can make it an ideal breeding ground for rhinoviruses.
Most research on rhinoviruses has primarily focused on examining how differences in body temperature affect the virus's ability to reproduce. However, more recent research is focusing on environmental factors that may increase the risk of developing rhinovirus infection.
One study looked at whether or not variations in temperature and humidity lead to a higher risk of rhinovirus infection. Researchers found that decreasing temperature and humidity over a 3-day period increased the risk of rhinovirus infection in participants. In the same study, researchers found that the majority of infections occurred at temperatures of 0°C or lower. Influenza viruses can also survive and spread more easily in cold, dry air. A study in guinea pigs suggests that the ideal temperature for the spread of the flu virus is 5°C.
However, the fever, chills, sore throat, cough, and runny nose we experience when we have a cold are actually the result of our immune system reacting to these viruses. When the virus enters the respiratory system, through the nose or mouth, it seeps into our cells and begins to multiply. Our immune system responds by sending out an army of immune cells and chemicals that trigger the inflammatory response, which causes many of the symptoms we associate with a cold or flu.
It is true that the cold can play a small role in the increase in colds and flus during the colder months. One way our immune system fights viruses is by suppressing their ability to replicate, but some studies have shown cold weather can slow this response.
Indeed, studies have shown that during the winter months, many people have less vitamin D due to reduced exposure to sunlight. And several researches suggest that vitamin D plays a vital role in maintaining the immune system. Additionally, a 2015 study found that exposing airway cells taken from mice to lower temperatures decreased the cells' immune response against a mouse-targeting rhinovirus.
Finally, breathing in cold, dry air causes the blood vessels in the upper airways to narrow in an effort to retain heat. This can prevent white blood cells from reaching the mucous membrane, making it harder for the body to fight infections.
Additionally, people with asthma may be at a higher risk of getting sick because cold air can worsen chronic disease, causing more asthma attacks. In this weakened state, the lungs may be more susceptible to infection. In fact, chronic conditions in general, such as diabetes, heart disease, and lung disease, all put you at greater risk for colds and flu.
Finally, in cold weather, some of our nose's natural defenses can be weakened. Cold air can dry out and crack interior tissues, creating an entryway for viruses, and the tiny hairs (cilia) inside our noses that sweep up pathogens may not be as effective in cold weather. That being said, regardless of the weather, infections like colds and flu do not occur without the presence of a pathogen.
The biggest problem with the colder months is that our response or adaptation to cold weather can make us more vulnerable to infections. For example, cold weather can cause our noses to run a little more, so more nasal secretions (and germs) have the potential to spread. Also, if people with colds or flu are on the go, they are more likely to cough or sneeze, spreading the virus through the air.
The dry air usually generated by heaters provides an excellent airborne medium for moisture droplets that are expelled during a cough or sneeze. They stay suspended longer in warm, dry air, creating a longer exposure period for people in that space. Being sick more often in cold weather therefore depends on a combination of several factors, and not on the temperature itself. In conclusion, it is the pathogens and the conditions in which they are brought to evolve that make you sick, but not the cold itself.
