The most common of the 200 viruses that can cause the common cold is the rhinovirus. It is highly contagious, entering the body through the mouth or nose. It spreads through droplets in the air when someone who has the virus coughs, sneezes, or talks. Sharing common objects such as utensils, telephones or toys, or shaking hands with an infected person will quickly spread the virus from the already infected person to the next person. People, including adults and children, with a weakened immune system are particularly vulnerable.

The season of the year can be a factor, too. The fall and winter, when people tend to congregate indoors and spend time more closely with others, can facilitate the spread of the germs that cause colds. In temperate climates that do not experience winter as a cold or freezing season, the rainy season is likely to produce a higher rate of colds for the same reasons.