Cold is caused by the cold viruses, which aren't visible to the naked human eye but they really are present everywhere, waiting for a chance to enter human and animal bodies.
Viruses are parasites that feed off human and animal cells in order to multiply. They are only about 20 to 300 nano-meters in size. (1 nano-meter = 1/1,000,000th millimeter). Cold viruses enter our body through mouth, nose or eyes.
So why is cold very common during col seasons? The answer is: The mucous membrane of the nose and throat has fine hairs called CILIA, growing on it. These act as filters and and trap the dirt and germs hat try to enter through nose and throat. During winter, the capillary vessels contract and the movement of the cilia slows down, thereby making them weaker.
The other reason is, during cold seasons, the air will be very dry. Dry air increases the movement of bacteria and viruses, and that is why colds are so common during winter. In short word, we say, " COLDS LOVE THE COLD! ".
