During the years of World War I, several severe typhus epidemics were seen in Erzurum and nearby cities. A total of 164 health officers, 125 of whom were physicians, struggled against the epidemic in the region but also they lost their lives due to typhus. Vaccination against typhus was one of the means of fighting the epidemic. However, there were some claims that a small group of Turkish physicians injected typhus-contaminated serum into Armenian civilians during World War I, and that this should be accepted as a form of biological warfare against Armenian civilians. The purpose of this article is to set out how, by whom, and on whom, and under what conditions the typhus vaccination was applied in order to reveal the truth in terms of the evidence found in historical documents. The typhus vaccine was prepared from blood taken from febrile patients affected by the disease. After the blood of the patients were defibrinated and inactivated at 60 degrees C for an hour, it was used. As the amount of blood needed to prepare the vaccine was so great, the amount of available vaccine was always insufficient to meet the demand. Hence, the prepared vaccine was only applied to those which had the higher risk of contracting typhus such as physicians and nurses. The vaccine prepared by The Third Army Health Commander Dr. Tevfik Salim was first applied to nine officers, five of whom were physicians, and among whom were Dr. Haydar Cemal and Dr. Salahattin on March 28, 1915 in Hasankale, Erzurum. Furthermore, the same vaccine was applied to people in the vicinity by Dr. Alaattin in Erzurum, Dr. Abdulhalim Asim in Bayburt, Dr. Izak in Sivas and Dr. Mihran in Hasankale. Ali Ihsan Sabis and Fevzi Cakmak, who were high ranking officers, were among those who volunteered to have the vaccination. The Third Army Health Commander Dr. Tevfik Salim ordered that the vaccine should not be applied without blood inactivation. Despite this order, Dr. Hamit Osman, who had a mental illness, applied the vaccination without inactivating the blood to some people. Among those were physicians of the Red Crescent Hospital together with soldiers who were nursing in the hospitals in Erzincan. Dr. Hamdi Suat inactivated the blood by leaving it at -16 degrees C for 24-48 hours, and instead of giving a single dose, he applied three-doses with 3-day-intervals, followed by a one more dose, which he called "the vaccine for absolute immunization" to the same people after 10-23 days. This "vaccine for absolute immunization" was actually typhus-contaminated blood which had not been inactivated. It should be noted that he injected himself with the same form of vaccine. In his article published in German in 1916 and in Turkish in 1917, he stated that he injected "the vaccine for absolute immunization" to some subjects 'condemned to death'. Dr. Haydar Cemal claimed, in a newspaper dated December 23, 1918, that the people reported as subjects 'condemned to death' were indeed Armenians, and that the innocent Armenians marked out for deportation were inoculated with the blood of typhus fever patients, and that he eyewitnessed all these events. As a result of his claims, the Interior Ministry demanded an immediate investigation, and at the end of that investigation it was understood that Dr. Haydar Cemal and Dr. Hamdi Suat had never worked together in Erzincan at the time Dr. Haydar Cemal claimed. All the claims were refuted by the investigating committee and nobody was charged. During a severe typhus epidemic, Turkish physicians injected the typhus vaccine for the purpose of "saving a life from the fire". The typhus vaccine was prepared using the available scientific knowledge of the time. No racial or religious discrimination against the people vaccinated had been proved. According to the sources, the claim that some Turkish physicians used the blood of patients with typhus as a means of biological warfare does not reflect the historical truth.