Microorganisms that cause disease in humans and other species are known as pathogens. The transmission of pathogens to a human or other host can occur in a number of ways, depending upon the microorganism.

A common route is via water. The ingestion of contaminated water introduces the microbes into the digestive system. Intestinal upsets can result. As well, an organism may be capable of entering the cells that line the digestive tract and gaining entry to the bloodstream. From there, an infection can become widely dispersed. A prominent example of a water borne pathogen is Vibrio cholerae, the bacterium that causes cholera. The contamination of drinking water by this bacterium is still at epidemic proportions in some areas of the world.

Pathogens can also be transmitted via the air. Viruses and bacterial spores are light enough to be lifted on the breeze. These agents...