Archaea

Archaea are a sparate domain than Bacteria and Eukarya. They lack murein in cell wall, and instead rely on ether lipids in their cell membranes. No clear examples of pathogenic archaea have been found, but they are often mutualists. One good example is the methanogens that inhabit the human and ruminant (cow) gut. They make us fart and cows belch methane, a greenhouse gas that is many times more potent than CO2, but they won't kill us directly, which is nice.

Extremophiles
The domain archaea contains renowned examples of extremophiles such as hyperthermophiles and halophiles. Hyperthermophiles thrive in temperatures of 80 to 120°C, and some halophiles can grow in saturated solutions of salt (36% salt by weight).