A third infected mosquito bite was administered to Maass in June 1901 during the continuing yellow fever trials in Havana.
By June 1901, Clara Maass had undergone a third deliberate exposure to a mosquito carrying the yellow fever pathogen. This continued participation in the experiments run by the U.S. Army's Yellow Fever Board in Havana, Cuba, reflected both her personal commitment to advancing medical science and the financial incentives — a modest payment — offered to volunteers willing to undergo such risky procedures.
The ongoing trials were central to confirming the transmission mechanism of yellow fever, a disease that had devastated military and civilian populations throughout the Americas for centuries, including major outbreaks in Philadelphia, New Orleans, and Havana itself. Prior to this research, theories of transmission ranged from contaminated bedding and clothing to miasma (bad air), and proving the mosquito vector theory required convincing, repeatable human evidence.
Maass's tolerance of repeated exposures without severe illness likely reinforced researchers' confidence in the experimental protocol, even as the risks compounded with each subsequent bite. Her case exemplifies the extraordinary risks nurses and other early medical volunteers took in service of scientific advancement during a period when human subjects research lacked formal ethical oversight. The knowledge gained from these repeated trials would soon prove pivotal — and costly — culminating in tragedy just months later.