Popular Boston physician, Dr. Michael W. Shannon, collapsed and died Tuesday after returning from Argentina, reports The Boston Globe. He was 55.

Shannon, the first Black full professor of pediatrics at Harvard Medical School, was thought of as one of the best pediatric toxicologists in the world, according to colleague Dr. Gary Fleisher, who serves as chairman of medicine at Children’s Hospital in Boston.

“He was extremely beloved. He was just a wonderful colleague. You could depend on him for absolutely everything,” Fleisher said.

Shannon was stepping off an airplane in New York, returning from a tango dancing trip he took to Argentina with his wife Elaine, when he suddenly collapsed at around 6:30 a.m., says a family friend. Doctors at nearby Jamaica Hospital Medical Center were unable to revive him. His cause of death is not yet known.

Shannon started dancing 30 years ago and even performed as a professional during his career. During their trip to Argentina, he had the time of his life, according to a friend.

“This was the trip of their lifetime,” family friend Nancy Teumer told the paper. “They had the time of their life, dancing all day and all night.”

Shannon leaves behind two college-aged children; son Evan, 21 and daughter Lila, 20. Both currently attend Stanford University.

During his career, Shannon advocated for improved medicine for children and spoke in front of the Food and Drug Administration last year against children being given cold medicine.

“When a treatment is ineffective, its risks – unless zero – always exceed its benefits,” he said.

Shannon’s dedication to his job was immense.

“Michael never slept. He was [always] doing something, whether it was in the arts or in the medical field, helping children or being with his family. He was just an incredible person, and all that knew him were touched by him,” Fleisher said.

He was also active in the community, starting a youth group for teens at his church in Brookline, Mass, reports the paper.

Funeral plans have not been finalized.