There are microbes in all three domains.

• 13.1 Not all microbes are closely related evolutionarily.
• 13.2 Microbes are the simplest but most successful organisms on earth.

Bacteria may be the most diverse of all organisms.

• 13.3 What are bacteria?
• 13.4 Bacterial growth and reproduction is fast and efficient.
• 13.5 Metabolic diversity among the bacteria is extreme.

In humans, bacteria can have harmful or beneficial health effects.

• 13.6 Many bacteria are beneficial to humans.
• 13.7 This is how we do it: Are bacteria thriving in our offices, on our desks?
• 13.8 Bacteria cause many human diseases.
• 13.9 Sexually transmitted diseases reveal battles between microbes and humans.
• 13.10 Bacteria’s resistance to drugs can evolve quickly.

Archaea exploit some of the most extreme habitats.

• 13.11 Archaea are profoundly different from bacteria.
• 13.12 Archaea thrive in habitats too extreme for most other organisms.

Most protists are single-celled eukaryotes.

• 13.13 The first eukaryotes were protists.
• 13.14 There are animal-like protists, fungus-like protists, and plant-like protists.
• 13.15 Some protists can make you very sick.

Viruses are at the border between living and non-living.

• 13.16 Viruses are not exactly living organisms.
• 13.17 Viruses are responsible for many health problems.
• 13.18 Viruses infect a wide range of organisms.
• 13.19 HIV illustrates the difficulty of controlling infectious viruses.