A deadly bacillus bred by the aseptic, artificial environments of 23rd-century life, carried without symptoms by Dr Sevrin, who is therefore barred from primitive worlds.
Synthococcus novae is described in the Original Series episode The Way to Eden. McCoy explains that the organism is a product of the sterilised, engineered conditions in which Federation citizens live; immunisation protects those who stay within that society.
Dr Sevrin, leader of a band of young idealists seeking the planet Eden, is found to be a carrier. He is not ill himself, but he could start a lethal epidemic among any people without modern medicine, and so he is forbidden to travel to undeveloped planets.
Sevrin knows this and goes anyway, seizing the Enterprise to reach Eden. Spock considers him insane, and the diagnosis sharpens the episode's irony: the man who rejects technology is himself a creation of it.