Based on my own personal experience, there exist plain eye invisible particles that carry over and do infect common cold from a sick to healthy man. I don't care if they are fungi, bacteria, viruses or malicious invisible human body parts, but something certainly exists and it works.

The healthy person can be immune (not necessarilly with specific antibodies, can be generally immune to "everything"), and resist the attack of the invisible particles, but when he is not immune (the same person, different time, food and other stuff), he will stay healthy if he is isolated from those particles, and will get sick if he gets them. Tested on myself a million times, no chance of mistake and no controls missing.