A common cold is an infection that is introduced by the respiratory track; the nose and upper respiratory track. It can either be introduced as an airborne infection or as an infection by the secretions with coughing and is very contagious.
It is mainly spread by being in a closed community; a closed space where the secretions are transmitted from one to the other; namely spit, nasal secretions, as well as mucus. Not necessarily breathing on one another.
A common cold usually starts as a runny nose. As the infection moves into the respiratory track, the larynx gets infected and it develops into croup. Soon afterwards it will move down into the trachea and you would get a cough and it starts to become productive.