I stopped watching Star Trek: Voyager before the introduction of Species 8472. I am watching SFDebris's Opiniated Voyager Episode Guide and stopped to look up information on the Maquis which, in the manner of Wikipedia, led through a bunch of links to Species 8472.
The Undine (as they are also called) are from a different dimension of "fluid space". They have a very complex genetic structure, possessing tri-helical DNA. Their cells can duplicate incredibly rapidly, acting as a virulent infection when introduced into other creatures (as happens when one is attacked by an Undine's claws).
Some time ago, I wrote a post about a troll infected with a virulent disease. The troll's regeneration protected it from any detrimental effects of the infection but allowed the rapacious disease to continually multiply and, thus, the troll served as a very effective carrier for the disease.
If I cared to transform that idea from a singular troll to a race of creatures, the Undine would be the first place to look.