THE MICROBIAL MENACE

Known Pathogens

Infectious diseases are caused by microorganisms such as bacteria and protozoa, by larger organisms such as tapeworms, and by viruses. While they carry genetic information, viruses are not fully functioning cellular organisms. Any list of known pathogens is a work in progress, since new pathogens are constantly arising. The result is an evolving microbial landscape in which a new and potentially virulent pathogen might appear any time and in any place.

Pathogens — associated with known diseases.

Changing environmental factors — from sanitation to overcrowding or increased travel — can also affect the spread of infectious disease. The rapid rate at which microbes reproduce is also a factor. In the space of a century, humans might produce as many as six or even seven generations. A bacterium (such as Escherichia coli) that reproduces every 20 minutes might produce more than 2.5 million generations in the same time period, each capable of change by genetic recombination or mutation.

Pfiesteria piscicida: pathogen or not?
In 1997, a public health crisis arose in Maryland when water-skiers, fishermen, and others in the Chesapeake Bay region started to develop mysterious skin lesions, short-term memory loss, and other neurological problems. Very quickly, the culprit was identified as P. piscicida — a marine organism related to the creature that causes the red tide. Discovered in the late 1980s, P. piscicida is a protist that causes large-scale fish kills by releasing a potent toxin to make fish lethargic and destroy their skin. With further study, it became evident that in sufficiently high concentrations, the toxin can become airborne in small droplets and harm people. Thus, P. piscicida is an interesting example of what may at first glance look like an infectious disease but may be something else entirely — in this case, a purely chemical effect.