Protozoa are single cells, and the way one gets in tells you how to keep it out. Walk each parasite around its life cycle. Then switch prevention measures on and see which link in the chain actually breaks.
Plasmodium
Press Next stage to walk the cycle.
Plasmodiumcauses malaria
Pick a parasite
Break the chain
Turn measures on. Each one cuts a particular link, and some of them do nothing at all against this parasite.
Nothing switched on yetPick one or more measures above.