Quarian

The quarians are a nomadic species of humanoid aliens known for their skills with technology and synthetic intelligence. Since their homeworld Rannoch was conquered, the quarians live aboard the Migrant Fleet, a huge collection of starships that travel as a single fleet.

Approximately three hundred years ago the quarians created the geth, a species of rudimentary artificial intelligences, to serve as an efficient source of manual labor. However, when the geth gradually became sentient, the quarians became terrified of possible consequences and tried to destroy their creations. The latter won the resulting war and forced their creators into exile.

Now the quarians wander the galaxy in a flotilla of salvaged ships, secondhand vessels, and recycled technology called the Migrant Fleet, seeking either a suitable new homeworld or a way to oust the geth from their native planet. Due to the nature of their homeworld and living in the artificially sterile environment of the fleet, the quarian immune system is very weak. They must wear environmentally sealed suits at all times- any exposure to foreign microbes causes intense illness and possible death in a quarian.