De Lellis established the Order of Clerks Regular, Ministers of the Infirm (abbreviated as M.I.), better known as the Camillians. His experience in wars led him to establish a group of health care workers who would assist soldiers on the battlefield. The large, red cross on their cassock remains a symbol of the Congregation today. Camillians today continue to identify themselves with this emblem on their habits, a symbol universally recognized today as the sign of charity and service. This was the original Red Cross, hundreds of years before the International Red Cross and Red Crescent Movement was formed.
During the Battle of Canizza in 1601, while Camillians were helping with the wounded, the tent in which they were tending to the sick and in which they had all of their equipment and supplies was completely destroyed and burned to the ground. Everything in the tent was destroyed except the red cross of a religious habit belonging to one of the Camillians who was ministering to the wounded on the battlefield. This event was taken by the Camillans to manifest divine approval of the Red Cross of St. Camillus.