Over a hundred have been identified so far. A virus is a disease-producing agent so small that it goes right through the very fine filter that stops bacteria. These cold-causing viruses are present in great numbers in your nose and throat when you have a cold. They are carried on the droplets you expel when talking, coughing, or sneezing. Though a virus causes your cold, a number of conditions seem to increase your susceptibility.
People appear to catch colds mainly indoors.
Certainly they get more of them during the colder months. This is probably because of more indoor living in proximity to other people's cold viruses, with closed windows and lack of circulating air allowing the viruses to remain alive and on hand. On the Apollo 7 space flight, you may remember, first the captain and then, within a few days, the crew came down with colds. From one to three days after the virus takes hold in your body, the symptoms appear. The first hint is usually scratchiness in the throat.