"rare disease x which has 80% mortality within 1 week"
"DRUG B, cures only 70% of patients (the remaining 30% still die) but is very safe"
Drug B "cures" (by definition, eliminates the disease) in 70%, but why would the other 30% have to die? After all the disease has an 80% mortality rate, not 100%! The number who should die if 70% are cured is 30% x 80% = 24% only. So 76% survive if they take drug B.