With then Lieutenant Charles Franklin Craig, Ashburn made the link that mosquitoes were involved in the transmission of Dengue fever.

He accepted a Regular Army commission as an assistant surgeon on January 9, 1899, and was promoted to captain in the Medical Corps on December 12, 1903, and major on June 24, 1908.

[3] The board's findings were released as Experimental Investigations Regarding the Etiology of Dengue Fever, with a General Consideration of the Disease with Ashburn and Craig as the co-authors.

[6] In 1913, he was detailed to preside over the Army board for the study of tropical diseases in the Philippines and at Ancon, Panama.

[8] He commanded the Medical Officers Training Camp at Fort Benjamin Harrison, Indiana in 1917.

[11] Ashburn was made an officer in France's Legion of Honour for his services during World War I.