The common cold can be spread through just one sneeze, and that single sneeze can let fly as many as 100,000 germs traveling at about 100 miles (161 kilometers) per hour. And although in the 1960s and '70s we believed we were safe from germ transmission as long as we had buffer of 3 feet (1 meter) from said sneeze, recent research suggests the droplet-spray radius is actually five to 200 times larger than that [sources: Hatfield, Bourouiba].
Millions of people suffer through an average of two or three colds every year, which, when you think about how each round lasts anywhere from a week to 10 days, is nothing to sneeze at — and that's with an understanding of how it's spread. We used to think that the common cold was caused by being in a cold environment. We got that one wrong, didn't we?
Infectious diseases are bacterial, viral, parasitic and fungal infections that spread among us through direct, person-to-person or indirect contact, not through witchcraft, punishment for sin, a problem of the body's "humours" or any other mystical force. Direct contact transmission refers to diseases, such as human papillomavirus (HPV) and the Ebola virus, that need physical contact to spread from person to person (or, for instance, direct contact with an infected animal, such as a bite from a Borrelia burgdorferi-infected tick leading to Lyme disease). Indirect contact, on the other hand, doesn't require any skin-to-skin contact; MRSA, for instance, is able to linger on a keyboard for weeks after an infected person typed on it. And measles, spread through airborne transmission, similarly lingers for hours outside the body, except instead of remaining on objects, these germs remain suspended in the air around us.
But let's set all that aside and look at the more supernatural and moral meanings we once used — and sometimes still do — to explain certain diseases.