In this semi-tropical region, he was as enthusiastic in his scientific research as he had ever been. From Florida he was transferred to Governor's Island, off Boston, where he was able to bring into requisition the results of his researches in Southern Florida. His valuable service was shown in the prevention of yellow fever from infected ships from the tropics. While stationed at Governor's Island, Dr. Guild wrote a treatise on yellow fever, which was published by the government. He was the first to insist stoutly that yellow fever is infectious, though not contagious, a theory then new, but now accepted. Nothing relative to the health of the army escaped his trained eye. About the time about which we are now writing, a meat biscuit which was issued to the army, became quite popular, but he condemned it as unhealthful, and was instrumental in inducing its discontinuance. From Boston, Dr. Guild was assigned to duty on the Pacific Coast, where Gen. Albert Sidney Johnston commanded the Pacific Coast division of the regular army. Dr. Guild's official duties were such as to enable him to witness many scenes of Indian warfare in the Far West. It was while he was serving on the Pacific Coast that the rupture came between the North and the South. Promptly sacrificing his accumulated means, and the popular and lucrative position which he had gained in the army, he resigned, turned his face southward, visited his old home in Tuscaloosa, and repaired to Richmond, where in July, 1861, he was appointed a surgeon in the Confederate army. The following month, he was sent by the Confederate government on a tour of inspection of the hospitals throughout the South. On his return to Richmond, Dr. Guild was assigned to duty at the front, where his relations with Gen. Joseph E. Johnston became the most intimate, and the families of both constituted a charming circle of army society. Dr. Guild was among many others who insisted that General Johnston was among the greatest strategists of either army. When General Johnston was wounded at Seven Pines, and General Lee took command, one of the first inquiries of General Lee was: "Where is Dr. Guild? Tell him to report to me at once." It was on the battle field of Seven Pines that Dr. Guild was made medical director and chief surgeon of the army of Northern Virginia, which position he held to the close of the war. This position placed him on General Lee's staff, and from that time till the close of the long and bloody tragedy, Dr. Guild sustained the closest personal relationship with the greatest southern chieftain.