A viral crèche is a point in the life-cycle of some viruses. When a virus infects a larger organism, a viral crèche consists of foreign body in the infected creature, affecting changes in life processes.

Doctor Beverly Crusher thought the effects of Tarchannen III species parasite acted similarly in the body to a viral crèche, although Alyssa Ogawa stated it would have to be very small to avoid detection. (TNG: "Identity Crisis")

It wasn't yet stated that there was a parasite involved, but Crusher deduced that the parasites effects were caused by something like a "viral crèche" ("crèche" refers, in biology, to an early stage of life, or the location thereof). It wasn't further described, but from the way she formed her sentence, it revealed a viral crèche is a foreign body that, while containing one or more viruses, effects changes like those described.