Dr. Reed had prepared, as I have above mentioned, a very comfortable camp at an isolated point near Camp Columbia, well separated from all other dwellings. This camp was kept under military guard, so that no one could come and go without Dr. Reed’s knowledge. Here he placed his non-immune volunteer Spaniards whom he had gotten from Havana, and kept them under observation for two weeks, taking their temperatures every day so as to be sure that they had not contracted yellow fever before they went out to the camp. At this point he made another important discovery in the mode of yellow-fever propagation. He found that the mosquito herself had to wait from ten to fifteen days after she had bitten a man sick with yellow fever before she herself conveyed the disease. He found that the mosquito for the first week or ten days after she had bitten the yellow-fever patient was entirely harmless though she fed freely on non-immunes. But after the twelfth or fourteenth day she would give the disease to every non-immune whom she bit. I have often seen the non-immune doctors and nurses at Las Animas Hospital put their hands in the jars where infected mosquitoes were kept during the first seven days of their infection and allow them to draw their fill of blood, for the purpose of feeding them, but they would not think of doing this after the seventh or eighth day. Two of these nurses afterwards contracted yellow fever from allowing mosquitoes to bite them after the twelfth day, and one of them, Miss Mass, died from the disease so contracted. Dr. Carlos Finlay, in his many experiments on the human being, was unaware of these two facts with regard to the transmission of yellow fever: first, that the mosquito could only become infected by biting a human being within the first three days of his disease; and second, that she could only become infectious, that is, transmit the disease, when some twelve or fourteen days had passed since she had bitten the sick man. Dr. Finlay put a great many of his mosquitoes to the sick man after the third day, and in no case did he apply his mosquito to the non-immune twelve or fourteen days after she had bitten the infected person.