Maass's final experimental exposure to an infected mosquito in August 1901 led to a severe, ultimately fatal case of yellow fever.
In August 1901, Clara Maass underwent her fourth and final deliberate exposure to a mosquito infected with yellow fever as part of the U.S. Army's ongoing research program in Havana, Cuba. Unlike her earlier, milder reactions, this exposure triggered a severe and rapidly progressing case of the disease.
Some historical accounts suggest Maass may have specifically agreed to this final trial partly in hopes of testing a potential immunizing serum, and partly for the promised stipend, which she intended to use to help support her family and further her education. Whatever her precise motivations, this exposure proved catastrophic.
Within days of the bite, Maass developed the classic severe symptoms of yellow fever — high fever, jaundice, hemorrhaging, and organ failure — and her condition deteriorated quickly despite medical attention. Her death shortly afterward sent shockwaves through the medical and nursing communities and the American public, sparking outrage over the ethics of using human volunteers, and especially women, in such dangerous experiments.
The tragedy directly contributed to the end of human experimentation trials for yellow fever research and reinforced the shift in public health strategy toward mosquito eradication and environmental control, rather than attempts at direct immunization through deliberate infection. Her fatal exposure remains one of the most cited cases in the history of medical research ethics.