After graduating Carroll studied bacteriology under Dr. William H. Welch at Johns Hopkins Hospital and assisted Walter Reed in pathology laboratories.

[2] In 1900 he served as an American physician and a member of the Yellow Fever Commission in Cuba, along with Walter Reed, Jesse William Lazear, and Aristides Agramonte.

The exact origin of Carroll’s yellow fever infection, whether from the mosquito bite or prior exposure to affected patients, remains uncertain.

[3] After a trip to Washington D.C., Carroll returned to Cuba for additional studies in which he proved that blood from active cases of yellow fever contained sub-microscopic infective agents.

[4] In 1904, with permission from Army Surgeon-General Robert Maitland O'Reilly, Carroll tested an oral typhoid fever vaccine on himself and 12 other volunteers from the military.