There are over 200 different strains of “cold” viruses, mainly made up of rhinoviruses (up to 50%). The average adult in the US will get 2-4 colds per year. Up to 25% of infected people won’t show any symptoms at all. Most won’t get a fever, and if you do, it will be low-grade- around 100 degrees Fahrenheit. These viruses primarily won’t transmit through the air. Instead, people become infected by coming into direct contact with someone who has an infection, or touch something that an infected person touches.
Influenza, however, is a much more sinister beast, affecting approximately 10% of the US population every year. This virus tends to start with sudden onset of a higher fever between 100-104 degrees Fahrenheit. It then progresses into chills, headache, muscle aches and a loss of appetite. Approximately 20,000 people a year die from the flu.
So why the increase in these illnesses in the winter? It seems this knowledge has been around for many centuries. The word influenza comes from an older Italian phrase “influenza di freddo” or “influence of the cold”.
The flu-season usually ranges from November to March in the northern hemisphere (the coldest months) and May to September in the lower. In fact, in tropical climates, there tends to be extremely low incidences of flu and certainly no true “flu season”.
There are several contributing factors to why cold temperatures increase influenza infection rates, all of which seem to be well known and promoted by health officials in numerous publications. The most common is that people tend to stay indoors when the temperatures get colder. This allows people to be in closer contact with each other and therefore makes it easier to pass the virus from person to person. Another contributing factor could be that in large parts of the country children are going back to school and interacting more with their fellow infected. In fact, most epidemics can be traced back to children.
The answer to this riddle lies in how the influenza virus reacts to temperature and humidity. The virus is extremely stable in colder temperatures, 41 degrees Fahrenheit optimally. The warmer you go, the less stable it becomes. Around 86 degrees, the virus isn’t transmitted at all.
Humidity also plays a very important role. Influenza is primarily transmitted on the droplets from your respiratory tract. The more humid the environment, the more water is available for those droplets to “pick up”. The heavier the droplets become, the faster they will fall to the ground and out of the way of our mucus membranes. In drier environments, those death droplets hang around in the air longer for others to breathe in. In fact, one study showed that the virus was best transmitted at a humidity of 20% and not transmitted at all once the humidity reached 80%.