25.11: Sexually transmitted diseases reveal battles between microbes and humans.

To many microbes, the human genitals and reproductive tract represent a desirable place to find shelter, nourishment, and opportunities for reproducing and dispersing. Unfortunately, these microbes can cause problems for humans in the form of sexually transmitted diseases (STDs). STDs produce symptoms of varying severity, from mild to extreme discomfort to sterility or even death. It is estimated that, worldwide, more than 300 million new cases of STDs occur each year.

Sexually transmitted diseases are caused by bacteria, viruses, fungi, protists, and even some arthropods. The organisms are passed from the mucous membranes (of the genitals, as well as of the anus and mouth) of one individual to those of another during sexual contact; sometimes they can also be transmitted by needles used for drug injections.

Some of the most common STDs, with their symptoms and treatments, are listed in FIGURE 25-22. Although most are curable with antibiotics, antifungal drugs, or anti-protozoan drugs, two characteristics of STDs make them nearly impossible to completely eradicate from a population: (1) their symptoms may be mild or completely absent at times, causing many people to unwittingly pass an infection to their partners, and (2) to prevent reinfection, both partners must be treated simultaneously. Furthermore, because most microbes have such high reproductive rates, populations of a microbe can evolve quickly and become resistant to existing drugs, reducing the long-term effectiveness of treatments. Consequently, the treatment of STDs represents one of the most pressing public health issues in the world today.