There's a decent Stargate Atlantis episode along these lines called Miller's Crossing. The chief scientist McKay gets kidnapped along with his sister (herself a

There's a decent Stargate Atlantis episode along these lines called Miller's Crossing. The chief scientist McKay gets kidnapped along with his sister (herself a brilliant scientist) by a billionaire trying to use alien nanite technology to cure his daughter of leukemia. In order to incentivize McKay to cooperate and fix the nanite coding, the guy injects McKay's sister with the nanites. McKay figures it out but the daughter ends up dying because, although the nanites cure her leukemia, they also cure an undiagnosed heart murmur by stopping her heart and repairing it thus depriving her brain of oxygen and leaving her braindead. They didn't have much time to explore the philosophical implications (sadly far too common for these scifi shows) but the rest of the episode is a race to disable the nanites before they try to "fix" the sister's epilepsy, including breaking her bones to create work for the nanites. The only type of nanites in the SG universe that could repair a human being without causing crippling unintended consequences were hyper-intelligent replicator nanites that were hell bent on destroying humanity because they had abandonment issues.