13.8: Bacteria cause many human diseases.

The number of pathogenic (disease-causing) bacteria is very small compared with the total number of bacterial species, but some pathogens kill millions of people annually, despite advances in medicine and sanitation. Some bacteria are always pathogenic (such as those that cause cholera, plague, and tuberculosis), but others (the ones responsible for acne, strep throat, scarlet fever, and “flesh-eating” necrotizing fasciitis) are normal parts of the communities of bacteria that live in or on humans. These bacteria in our “normal flora” become pathogenic only under special circumstances.

The cholera epidemic that devastated London in 1854 became a milestone in epidemiology (the study of the occurrence of disease outbreaks), when Dr. John Snow identified the Broad Street pump as the source of infection by mapping the pattern of deaths (FIGURE 13-11). When Dr. Snow persuaded the authorities to shut down the pump by removing the handle, new cases of cholera in the area dropped sharply.

Cholera epidemics are a continuing threat, especially in areas without effective sanitation. Epidemics of cholera are occurring now in parts of Iraq where the sewage systems have been destroyed and in Haiti, following the earthquake of 2010. The strains of cholera found in these areas are especially potent; as the bacteria multiply inside their hosts, they cause severe diarrhea with a massive loss of water. Victims of these severe strains of cholera are incapacitated and soon die, but before they die they release billions of cholera bacteria in diarrhea—an effective strategy for the bacteria, which contaminate the water used for drinking or bathing and rapidly infect new victims.

Streptococcus pyogenes is a normal part of the bacterial community of your nose and mouth. Normally it is harmless, and many people never experience any of the diseases it can cause. But when the population of S. pyogenes is not held in check by competition with other members of the bacterial community, it can become a pathogen. Strep throat (more formally known as “streptococcal pharyngitis”) is the most common disease caused by S. pyogenes; it produces a severe sore throat and a distinctive rash of red abscesses with white pus at the top of the throat (FIGURE 13-12). Some strains of S. pyogenes produce a toxin that is released into the bloodstream and produces a red rash that spreads across the skin—the disease called scarlet fever. Most threatening of all is an infection caused by strains of S. pyogenes that have a toxin that allows them to enter body tissues, where they produce necrotizing fasciitis. These strains of S. pyogenes are known by their well-earned name, “the flesh-eating bacteria.”