The common cold is the world's most widespread illness, which is probably why there are more myths(,) about it than any of the other illnesses.        The most widespread mistake of all is that colds are caused by cold.They are not.They are caused by viruses passing on from person to person.You catch a cold by coming into contact, directly or indirectly, with someone who already has one.If cold causes colds, it would be reasonable to expect the Eskimos to suffer from them forever.But they do not.And in isolated Arctic regions explorers have reported being free from colds until coming into contact again with infected  people from the outside world by way of packages and mail dropped from airplanes.        At the Common Cold Research Unit in England, volunteers took part in Experiments in which they gave themselves to the discomforts of being cold and wet for long stretches of time.After taking hot baths, they put on bathing suits, allowed themselves to be doused  with cold water, and then stood about dripping  wet in drafty  rooms.Some wore wet socks all day while others exercised in the rain until close to exhaustion.Not one of the volunteers came down with a cold unless a cold virus was actually dropped in his nose.        If cold and wet have nothing to do with catching colds, why are they more frequent in the winter? In spite of the most painstaking research, no one has yet found the answer.One explanation offered by scientists is that people tend to stay together indoors more in cold weather than at other times, and this makes it easier for cold viruses to be passed on.        No one has yet found a cure for the cold.There are drugs and pain suppressors such as aspirin, but all they do is to relieve the symptoms. According to the passage, colds are commonly seen in winter because  _  .
there is great difference between indoor and outdoor temperature viruses can go into people's warm bodies more easily in winter staying together indoors makes it easier for viruses to pass on people are usually weak because of the extreme cold in winter