From Creatures Wikia
In Creatures 1, they randomly infect a norn or grendel with one antigen. Later, after the bacteria themselves have evolved, they randomly infect a norn or grendel with an unknown toxin, which may include more than one antigen, one or more histamines, and other toxins. Players that wish to avoid this evolution can export their norns and reset their world using the Emergency Kit.
In Creatures 3, you cannot see the bacteria without aid. The Hoverdoc can provide this in the form of a bacteria-detection spray, or you can use VisiBug by Hausmouse. The HoverDoc and Containment Chamber can also provide bacteria-killing spray. In DS, you can use the Bacteria Button by Moe to see the bacteria.
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