My Body, My Laboratory

Those two seemingly opposing motivations — self-promotion and altruism — have long driven the practice. U.S. Army physician Walter Reed and his three junior doctors, James Carroll, Aristides Agramonte and Jesse Lazear, are probably the most mythologized self-experimenters in U.S. history for their efforts to uncover the cause of the yellow-fever outbreak that ravaged troops during the Spanish-American War. Building on the work of another physician, Stubbins Ffirth, who drank the blackened vomit of yellow-fever survivors to prove that the disease was not contagious, Reed's doctors set out to prove that an insect bite, rather than person-to-person transmission, was how the virus spread. On a field trip to Cuba, the trio allowed mosquitoes to feast on their bodies. Carroll later wrote that the experiments went ahead because each of the doctors was "willing to take a soldier's chance." Lazear died during the experiment, and Carroll suffered long-term complications that led to his death at 53.

To some supporters of experimenting on oneself, such selflessness should underpin all scientific inquiry. Lawrence Altman, a physician and medical journalist, argues in his book Who Goes First? that researchers should take part in every medical trial — even if it is a large study — to avoid charges of elitism. "No man's life is worth more than another's," he writes. Other proponents go further, arguing that trying things out by oneself cuts costs and speeds development. "If you succeed with yourself, then you can go on to larger trials," says Allen Neuringer, a psychology professor at Reed College in Oregon. (Read TIME's cover story "Healing the Hurt.")

That's exactly what happened for David Pritchard, an immunologist at the University of Nottingham in Britain who was studying autoimmune disorders such as multiple sclerosis and severe allergies. From his work as a young man in Papua New Guinea, Pritchard was struck by the near total absence of such conditions in the developing world. That led him to become a proponent of the hygiene hypothesis, the idea that by successfully scrubbing out the bacteria and parasites that human bodies have evolved to battle, people in rich nations have inadvertently thrown their immune systems out of balance. That can sometimes cause the immune system to attack the body. Studies in mice suggested that infestation with hookworms can suppress autoimmune disorders. By 2004, Pritchard hoped to test whether something similar happened in humans, and there was only one way to start: by allowing a batch of worms to nest in his intestines.

The first step was to determine what would be a safe dose of hookworms for an adult. In small numbers, the parasite does not cause symptoms, but severe infestations kill 65,000 people a year and sicken hundreds of thousands more. Pritchard and his assistants randomly assigned various dosage levels to each team member, and each self-infected with the amount they drew. They learned that 10 hookworms was the upper limit for safety. Pritchard, unfortunately, was assigned a dose of 50 and spent five days in agony before receiving medical treatment. Nonetheless, the study helped secure approval and funding for a larger trial, which is ongoing, on patients with multiple sclerosis. (See the robots helping with surgeries and checkups.)

Pritchard's work had the support of his university, but many scientists turn to self-experimentation out of frustration. In 1982, Barry Marshall, an Australian gastroenterologist, became exasperated by his inability to convince the medical community that the common bacteria Helicobacter pylori, rather than stress, causes stomach ulcers, so he swallowed a beaker of the stuff. He developed severe colitis a few days later, but his theory gained acceptance. In 2005 he was awarded a Nobel Prize for his work.

For every scientist thinking of following Marshall's lead, it's worth remembering that not all roads lead to Stockholm. In 2009, Yolanda Cox, a 22-year-old pharmaceutical researcher, died after she and her physician sister injected her with an experimental drug in search of a way to slow the aging process.

See TIME's special report "How to Live 100 Years."