In general it's hard to make value judgements about which traits are positive and which are negative. Natural selection cares only for survival advantage of offspring in the immediate environment.

A good example is sickle cell trait, which provides a higher resistance to malaria, but a risk of a variety of other complications. In areas where malaria is a severe threat, the survival advantage of the higher resistance outweighs the other drawbacks. Medical intervention such as administering a malaria vaccine might reduce the threat of malaria such that there would be selection pressure against the sickle cell trait instead of for it.