An infectious disease is one that can be spread from person to person (or animal to animal). Where a medical condition is caused by a virus, bacteria, or parasite, their reproduction poses a threat to those in contact with the sufferer, or in the same surroundings. Sometimes there is an intermediate vector that spreads the disease, or it may be airborne, waterborne, or transmitted by direct contact.
EXAMPLE: The virus for the common cold is spread by airborne fluid droplets or by contaminated surfaces. Measles and chickenpox are spread by contact.