People always ask: What can I do to make a difference?

So many people in poor countries desperately need assistance. So many people in rich countries would like to help but fear their donations would line the pocket of a corrupt official or be lost in an aid bureaucracy. The result is a short circuit, leaving both sides unfulfilled.

That’s where Scott Harrison comes in.

Five years ago, Mr. Harrison was a nightclub promoter in Manhattan who spent his nights surrounded by friends in a blur of alcohol, cocaine and marijuana. He lived in a luxurious apartment and drove a BMW  but then on a vacation in South America he underwent a spiritual crisis.

“I realized I was the most selfish, sycophantic and miserable human being,” he recalled. “I was the worst person I knew.”

Mr. Harrison, now 33, found an aid organization that would accept him as a volunteer photographer  if he paid $500 a month to cover expenses. And so he did. The organization was Mercy Ships, a Christian aid group that performs surgeries in poor countries with volunteer doctors.