Common cold, as the name suggests, is the most frequently & commonly occurring illness around the world. It is a contagious, viral infectious disease of upper respiratory system. There are more than 100 types of viruses that cause common cold. That's why we seem to be unable to develop immunity against it nor have the scientist been able to develop an effective vaccine.
It is most commonly caused by rhinovirus or coronavirus. It is normal for children to have cold many times in an year (sometimes upto 10 times). This is because they are meeting each one of the virus for the first time. Gradually they build up immunity and get fewer colds as they older.