Parasites, in general, defend themselves from the body's immune system and weaken the body's defenses around them; other infections (usually bacterial) take root. And so, an infectious disease and a parasite pairs up. The bacteria will adapt and live part of its life cycle in the parasite. Kills off the parasite and the bacteria infection goes away.
In this case, Cryptocotyle Lingua and Botulinum.
Maybe...speculatively.
