maybe tape worm was a bad example, but there are all sorts of other parasites, like ones that attack muscles, bones, brain, skin, sinuses, whatever. the 'morphs could just direct themselves into the yeerk, leave some asexual eggs inside the yeerk and hightail it outta there. yeerk goes into the pool 3 days later with tons of parasites which spread to the fellow yeerks. it's combat war of the worlds style