Stephen W. Lagakos, a long-time Harvard biostatistics professor whom colleagues remembered for his scientific leadership and personal warmth, died in a car crash Monday.

His Toyota Highlander burst into flames after it swerved across the center line of Route 202 in New Hampshire and collided with a Chevy Malibu, leaving his wife Regina, his mother Helen, and the other driver dead at the scene. Lagakos was 63.

At the School of Public Health, Lagakos founded the Center for Biostatistics in AIDS Research and pioneered new methodologies to estimate life spans for AIDS patients and transmission of HIV from mothers to their children during pregnancy.

One colleague, Michael D. Hughes, said he still remembers how Lagakos helped him to “focus on the big picture, doing things that could effect public health and not focus on small details.”

“What he taught me to do was to think about pulling together a big collaboration of academic groups and pharmaceutical companies to really validate surrogate endpoints in the HIV setting,” Hughes said.

While his colleagues respected him for his eclectic approach to problem-solving, they all fondly recalled the influence Lagakos’s leadership had beyond the workplace. “I’m going to miss him dearly as a friend,” said Lee-Jen Wei, a professor of biostatistics, whose sons were also good friends with Lagakos’s.

“He wanted to know, in a very kind way, what was going on in your life, what interested you,” Hughes said. “He was interested in your family and things you liked to do.”

Hughes recalled the in-depth discussions about academic subjects that he and Lagakos frequently engaged in. “You didn’t have to worry about stepping on his toes,” he said. “You could have disagreements with him and still end up with a humorous discussion.”

“He was the kind of person who could not say ‘no,’” Wei added. “If I said ‘no’ to him, he’d probably want to figure out why. Most of the time, he was right.”

Lagakos also continually challenged himself outside of the classroom. This past summer, he took a solo motorcycle trip from Boston to the West Coast and back, stopping by Yellowstone and the Grand Canyon. He also recently climbed Mt. Washington, a 6,288-foot peak in New Hampshire.

During his time as department chair, Lagakos would invite some of his students and colleagues to hike to his cottage atop Mt. Monadnock for picnics.

“He was always there to celebrate work and to celebrate life,” said Victor De Gruttola, the current chair of the biostatistics department. “He mixed that capacity for enjoyment for life with his enjoyment for statistics, for getting things right, for being accurate, and he showed why it was exciting and not just a chore.”

De Gruttola, who had known Lagakos for 27 years, called his death “just a tragic day for us at Harvard.”

“I’ll miss the joy, his enthusiasm, and his tremendous generosity in helping people create careers,” he said.

Lagakos is survived by his two sons, David and Adrian.