[lumarlmp] Cure and disease

A scary bad thing is discovered.  Then we learn more about the scary bad thing: it is in fact a cure, treatment, or preventative for something worse, perhaps a disease.  Then, the disease is discovered, perhaps unleashed when the preventative was disabled because it was scary and bad.  Finally, the reason the disease exists is discovered, which is the most horrifying of all.

Somewhat similar to the plot of Halo, though the origins of the Flood have yet to be revealed.

Possible real world example: cancer.