Flesh-eating microbes were a type of micro-organism that feast on flesh from other species. At some point they had inhabited and infested the waters of the planet Desolation, playing a part in turning the world into a deserted ruin.
At some point the planet became the final stage in the Rally of the Twelve Galaxies. Former contestant and winner Ilin, who oversaw the rally, warned the current contestants, the Thirteenth Doctor and her companions to not even touch the water. Later, the Doctor herself confirmed these microbes were there. (TV: The Ghost Monument)