As you probably know, disease-causing agents (known as pathogens) are bacteria and viruses that proliferate inside our body. These microbes usually require a temperature of 37ºC, so it's rather unlikely that low temperature would encourage their thriving. A contradictory example comes from chickens. With a body temperature of 40-41ºC, chickens enjoy a higher resistance to bacterial infection, and it is well known among poultry farmers that wounds in chickens do not require antibiotic treatment, as infections are quite rare.
Adopted from Wikipedia
So the link between cold temperature and sickness is indirect. Let me explain: Our body is constantly exposed to different bacteria and viruses, and different people react differently to these pathogens. The outbreak of a disease depends on the health status of the person and the ability of that person's immune system to fight off the infection.
When it is cold outside, in trying to maintain a constant body temperature our body works very hard to overcome the temperature gaps between the indoor and outdoor environments. These efforts, which involve many physiological systems in the body, probably weaken the immune system, rendering the body more vulnerable to disease.
Another factor is that the more people become sick, the greater the chances for an outbreak of an epidemic. Cold weather keeps us in unventilated and warm spaces, making us more exposed to contagious agents in tiny airborne droplets (coming from coughs, sneezes or air conditioning systems). Therefore, maintaining a normal body temperature, ventilating closed spaces and avoiding contact with sick people can prevent an epidemic outbreak during the cold season.
Several studies have reported an increased morbidity among office workers also during the summer. One possible cause is working long hours in air-conditioned areas. Since air conditioning systems have become part of our daily lives only in the last 10-20 years, they constitute a rather new factor that may help spread pathogens in the summer.
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