A recent article in New Scientist is a perfect illustration of how mutations can be beneficial, how beneficial genes can spread rapidly, and how humans are still evolving.
The mutation is a single amino acid change that appears to provide complete resistance to kuru, a prion disease similar to CJD or mad cow disease. Kuru was passed on through ritual cannibalism in Papua New Guinea, when family members ate the brain of the dead out of respect. Of course, once the disease began to spread, more funerals would lead to more cannibalism, which lead to more disease.
The protective gene is thought to have arisen within the last 200 years, and spread rapidly due to the selective pressure of kuru on the community.