Whether he was doing medical research in a Harvard lab, playing the violin as a section leader in the Mozart Society Orchestra, or just hanging out, Peter Cai ’10 could always be found with a smile on his face, friends said yesterday.

The Adams House resident died suddenly Saturday morning, going into cardiac arrest near Weld Boathouse after completing the men’s race of the semiannual River Run.

“This is the worst type of tragedy,” said Adams House Master Sean G. Palfrey ’67. “It was unexpected, unexplained, sudden, and absolute. Despite having every possible resource available to help him, and despite the presence of two anesthesiologists who started resuscitation efforts immediately, there was nothing that could have been done.”

After being rushed to Mt. Auburn Hospital by paramedics, Cai was pronounced dead just before noon. The 20-year-old’s parents arrived in Cambridge later that afternoon from their home in Pittsburgh and spent the evening with their son’s friends in Adams House.

A shocked collection of friends remembered the aspiring doctor’s winning personality. “He was an inspiring person, deeply committed to science and medicine, but also very kind,” said Michelle C. Siao ’09, who worked in professor Thomas P. Maniatis’s lab along with Cai. “I’ll remember him for his great attitude towards life and work.”

Siao recalled a story that Cai’s mother told during Saturday night’s gathering about Peter receiving immunization shots as a baby. As the doctor came forward to administer the injections, the young Cai kept smiling.

“It’s fitting because he really did smile a lot,” Siao said.

Julia Ye ’10 played in the Mozart Society Orchestra with Cai, who was section leader for the violinists.

“He was always so enthusiastic about the orchestra, and he cared that all of the members were happy,” Ye said. “When we had new members, Peter took time to make them feel that they were already an integral part of the orchestra. I always admired the genuine desire he had to make people feel at ease.”

“It’s unthinkable that the world will not have a Dr. Peter Cai in the future,” Ye added.

Anupriya Singhal ’09 and Cai were certified as CPR instructors together two years ago before becoming co-directors of the Harvard Emergency Medical Services last summer. Although the group normally focused on training Harvard students, Cai decided to teach CPR to people in disadvantaged communities in Boston, Singhal said.

“He wanted to build a sense of community in the organization, and he wanted the volunteers to understand the importance of what they were doing,” Singhal said.

“You’d never guess from the way that he behaved that he was so brilliant,” said David Mattos ’09, who worked with Cai in the same laboratory and as a member of the Harvard Premedical Society, “He was very humble, very diligent, and very quiet. But not quiet in the shy sense. He spoke softly, but he was well aware of himself. He was certainly one of the smartest students that I knew at Harvard.”

There will be a public service at Memorial Church this Tuesday at noon to commemorate Cai’s life.

—Staff Writer Prateek Kumar can be reached at kumar@fas.harvard.edu.