Colds are contagious but only for the first week, according to MedlinePlus. Colds are easiest to pass in the first couple of days of onset of symptoms.
Colds are easy to pass because they are transmitted through water vapor in the air, so a person can catch a cold even if she never touches a person with a cold. The water droplets enter the air when a person sneezes or coughs, discharging the virus. As coughing and sneezing are common symptoms of a cold, it is readily released into the air once a person starts showing these symptoms, according to MedlinePlus.