Lewis, who is best known for his work with rapper Macklemore, is taking part in the 8th International AIDS Society (IAS) Conference on HIV Pathogenesis, Treatment and Prevention, with more than 6,000 scientists, community organizers and public health experts.
He's there to speak about 30/30, the charity he founded with his mother, Julie Lewis, who has lived with HIV for more than 30 years.
Julie Lewis says she was infected with HIV in 1984 when she got a blood transfusion while giving birth to her eldest child.
"I didn't know I was infected for six-and-a-half years. So I was diagnosed in 1990 when the blood bank called and said I might have got some infected blood."
The family lived in a very conservative community at the time, so they kept her HIV-status a secret. That changed in 1994 when they told their young children.
"When you tell a six year-old that you're HIV-positive, it's no longer confidential information," says Julie Lewis. "We were at the grocery store and (Ryan) was like, 'Does she know you have AIDS?' and I was like, 'Well, she does now.'"
Ryan Lewis says he didn't exactly understand what his mother was going through, but knew that she might not be around one day.
In the early 90s, an HIV diagnosis did not come with a bright future.
"I never thought I'd live this long," Julie Lewis says.
Ryan Lewis gives a lot of the credit for his mom's longevity to the treatment she received.
"At the end of the day, we were just extremely privileged to have medicine, especially in the early 90s," he says. "So we're doing something."
The Lewis' founded 30/30 in 2014, and has since used it to build health centres around the world, particularly in places with high rates of HIV.
Julie Lewis says they've built three clinics so far, and have plans to break ground on two more in the coming months.