For Mr. Castillejo, the experience was surreal. He watched as millions of people reacted to the news of his cure and speculated about his identity. “I was watching TV, and it’s, like, ‘OK, they’re talking about me,’” he said. “It was very strange, a very weird place to be.” But he remained resolute in his decision to remain private until a few weeks ago.

For one, his doctors are more certain now that he is virus-free. “We think this is a cure now, because it’s been another year and we’ve done a few more tests,” said his virologist, Dr. Ravindra Gupta of the University of Cambridge.

Mr. Castillejo also tested his own readiness in small ways. He set up a separate email address and telephone number for his life as “LP,” as he refers to himself, and opened a Twitter account. He began talking weekly with Mr. Brown, the only other person who could truly understand what he had been through. In December, Mr. Castillejo prepared a statement to be read aloud by a producer on BBC Radio 4.

After talking through his decision with his doctors, friends and mother, he decided the time was right to tell his story.

“I don’t want people to think, ‘Oh, you’ve been chosen,’” he said. “No, it just happened. I was in the right place, probably at the right time, when it happened.”

Mr. Castillejo grew up in Caracas, Venezuela. His father was of Spanish and Dutch descent — which later turned out to be crucial — and served as a pilot for an ecotourism company. Mr. Castillejo speaks reverently of his father, who died 20 years ago, and bears a strong resemblance to him. But his parents divorced when he was young, so he was primarily raised by his industrious mother, who now lives in London with him. “She taught me to be the best I could be, no matter what,” he said.

As a young man, Mr. Castillejo made his way first to Copenhagen and then to London in 2002. He was found to have H.I.V., the virus that causes AIDS, in 2003.