The common cold is the world's most widespread illness, which is a serious infection that man receives. 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 touching 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 separate Arctic  areas, explorers have reported being free from colds until touching again with infected people from the outside world by way of packages and mail dropped from airplanes. During the First World War, soldiers who spent long periods in the trenches, cold and wet, seldom caught colds. In the Second World War, prisoners at Auschwitz concentration camp, bare and starved, were astonished to find that they seldom had colds. 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 a long time. After taking hot baths, they put on bathing suits, allowed themselves to be with cold water, and then stood about dripping wet in a room. 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 then, cold and wet have nothing to do with catching colds, why are they more frequent in winter? Despite the most hard research, no one has yet found out the answer. One explanation offered by scientists is that people tend to stay together indoors more in cold weather than at other times, and that 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-killers such as aspirin, but all that they do is to reduce the symptoms. The writer thinks the cause of catching cold is due to  _  .
touching anyone directly touching anyone indirectly cold weather getting viruses from person to person