A 19th century image of the medical profession's attitude toward disease transmission is introduced through Ignaz Semmelweis' hypothesis: Infection can be caused by an external agent transmitted when physicians fail to sterilize their hands between patient examinations. Semmelweis' test of his hypothesis reduced the obstetrical death rate from 18% to almost 1%. However, he was degraded, defrocked, and driven to death by a profession whose emotions contradicted the evidence. Medical professionals like to believe they are not gullible, a trait defined as being easily duped. They rightly believe in their ability to avoid the error of accepting a result not supported by adequate evidence. They are not so free of the complementary error: refusing to accept a result that is supported by adequate evidence, which might be thought of as reverse gullibility. It is just as bad a logical error and just as serious a denial of the best medical care for our patients. Clearly Semmelweis, and later Louis Pasteur and others who were disbelieved, were correct. The profession was duped by its emotional adherence to current practice. To be truly professional, let us believe our evidence rather than our biases and not suffer from reverse gullibility.