During 1899 we had the most curious case in Havana of infection in the person of a nun, Sister Maria de los Angeles, a Dominican nun, and a native of France. She had come direct to New York from Europe, on the steamship _Celtic_, remained there two days, and then took the Ward Line steamer _Vigilancia_ for Havana. The trip from New York to Havana by this steamer occupies four days. The nun reached Havana September 8. She was feeling badly, though she did not give up her duties on that account. On September 11 she was taken sick with a chill, and died with well-marked yellow fever on the sixteenth. The circumstances were such that the Board was convinced that she had contracted the disease on board ship, and not in Havana. She must have gotten it in some way between New York and Havana. She could not have been infected in New York, as there had been no yellow fever there during the preceding twenty years. The steamer under discussion plied between Vera Cruz, Mexico and New York City, touching at Havana both going and coming. We found that the records of the ship showed that the last case of yellow fever aboard had occurred on the second trip before the one under consideration, antedating the time the nun was aboard by about a month. This was in the person of a passenger from Vera Cruz, who was taken off at the quarantine station at New York. He occupied the stateroom that afterwards was occupied by the nun on her trip to Havana. Such an instance twenty years before would very readily have been explained as an instance of infection from the room, but we now know that yellow fever can be contracted only through the bite of an infected female stegomyia mosquito. No other case had occurred on the ship during the month following this case, in the passengers from Vera Cruz. It is probable, therefore, that a stegomyia mosquito in this room must have bitten the passenger sick with yellow fever during the trip up to New York; that this mosquito remained in this room for nearly a month, biting the unfortunate nun soon after she got aboard. As no other cases occurred afterwards, it is quite probable that the nun killed the mosquito at the time of the biting. It is also probable that some immune occupied this room on the trip up from Vera Cruz to New York City, who would not be injured even if the mosquito bit him. On the preceding trip down from New York to Vera Cruz the person occupying the room would not have been injured by the biting of the mosquito, even if the person had been a non-immune, as the mosquito requires two weeks from the time she bites a yellow-fever patient before she herself becomes infectious. The preceding trip down was within this two weeks period of non-infectiousness in the mosquito. At first blush, the case certainly appeared very mysterious. How was it possible for a person coming from France via New York to Havana, to have yellow fever when she reached Havana!