I'm imagining how things were in reality, prehistoric times, and honestly, maybe people just learned "hey all those people got sick from that dead person, maybe the gods don't want us messing with it?" But that might be hard to implement, Nuggets with group learning capabilities. Another thing humans had was potential immunity, a super small chance to just not get sick, passed on to children. In a group of 30, 2 people would be around 7% population immunity, and enough to... repopulate the planet?
Oh, and post-illness immunity would be cool too. Per-Nugget antibodies.
Otherwise, people are right above, just make the Gravedigger/person who burns bodies a little more... motivated. (Even manual stimulation would work for me. "Suddenly a great voice boomed in my mind, and It sayeth upon me, 'Yo get that body or you gonna kill all your people bro'")
Best of development to ya!