Hospital-acquired infections have their own cultural vectors: doctors, nurses, and other attendants inadvertently transfer pathogens from their patients to their hands and then to other patients, either directly or indirectly through contamination of objects in the hospital. Such attendant-borne transmission is the major route for most serious infections acquired in hospitals, such as staphylococci, streptococci, enterococci, and Clostridium difficile, which can cause

life-threatening infections of the skin, lungs, and intestinal tract. Attendants usually do not become infected themselves partly because they are less vulnerable than their patients, partly because they will wash their hands before leaving the hospital, and partly because they may have generated some immunity to the hospital organisms. This history is as old as the hospitals themselves.

In the early 1850s the Hungarian physician Ignaz Semmelweis was perplexed by the fact that one out of eight healthy mothers-to-be who were admitted to the University of Vienna hospital to deliver babies were leaving in caskets. The "childbed fever" that killed the mothers shortly after delivery was characterized by sepsis, which in the mid-nineteenth century meant an invasion of the blood by rotten or putrid material. Today it means disease resulting from the presence of microbes or their toxins in the blood, a common consequence of hospital-acquired infections.

Semmelweis's concern turned to horror when he began to understand the reasons for the hospital's alarming statistics. He noticed that the women were dying from the same disease that physicians and medical students had been studying in the morgue in between giving pelvic exams. More important, he noticed that the women who received those pelvic exams were more likely to fall ill than those who did not. He concluded, decades before Pasteur and Koch established the germ theory of disease, that the doctors and medical students were inadvertently killing the mothers-to-be by transmitting some invisible agent of disease during the prenatal exams. After the women were removed to the morgue, the agents that killed them were inadvertently returned to the ward on the hands of medical personnel and then transferred from patient to patient during the pelvic exams. To break this cycle, Semmelweis introduced the practice of having hospital staff wash their hands with a chlorine disinfectant. This treatment was followed by an application of oil to the hands; the oil was intended to serve as a barrier to any organisms remaining on the hands—a mid-nineteenth century version of latex examination gloves.

Within a month, Semmelweis's brilliant intervention cut the mortality in his maternity ward from about one death for every eight admissions to about one death in thirty. He was rewarded in 1854 with the termination of his position. Semmelweis eventually returned to Hungary. Within a few months he was given a position at the University of Pest and was appointed a physician without pay at St. Rochus Hospital in Budapest, where he introduced his life-saving hygienic practices with some resistance, though less than he had faced in Vienna. He died young in 1865 from the same disease he sought to prevent, after accidently cutting himself during a postmortem examination of a woman who had died of childbed fever. In 1969, just over a century after his death, the oldest and most distinguished medical school in Hungary — the one that supported him after the University of Vienna spurned him—was renamed Semmelweis University of Medicine.

While Semmelweis was worrying about expectant mothers in Vienna and Budapest, Joseph Lister was having similar worries about his surgical patients in Glasgow. Half his amputees were dying of sepsis. Over a decade earlier, in 1851, while Lister was finishing his medical studies in London and Semmelweis was starting work at St. Rochus Hospital, the staunch Semmelweis supporter J. F. von Arneth arrived in Edinburgh to present Semmelweis's findings after having been rebuffed by leading Parisian obstetricians. In contrast with the other learned medical societies of Europe, the Medico-Chirurgical Society of Edinburgh responded favorably to the evidence. The 24-year-old Lister moved to Edinburgh a year later. Though Lister missed von Arneth's presentation, the responsive atmosphere in Edinburgh must have fostered the line of thinking that he would soon formulate. The son of a wine merchant, Lister knew of Pasteur's insights about the role of microbes in fermentation, and he made the connection between microbes and hygienic prevention of disease even before Pasteur. Lister became convinced that microbes were the culprits in his postamputation sepsis and that the transmission of the microbes could be blocked by the right hygienic intervention. Looking for a disinfecting agent that was suitable for destroying microbes on living tissue, Lister remembered that sewage had been treated with carbolic acid (now called phenol) in an effort to curb waterborne disease in Carlisle, England, about 90 miles south of Glasgow. On August 12,1865, the day before Semmelweis's life would be claimed by the bacteria he'd spent his career trying to control, Lister first used carbolic acid successfully, disinfecting a compound fracture in his surgery in Glasgow. He soon developed methods for disinfecting not just the patient but the surgical equipment and the room itself. Deaths in his wards dropped from nearly one out of every two amputees to less than one out of seven during the first three years that his disinfection procedures were in place.

Lister did not suffer the kind of ostracism that stymied Semmelweis, partly because Lister's techniques were more readily demonstrable, but also because he had friends in high places. Pasteur in particular praised Lister's work, much as Lister praised Pasteur's. With support from Pasteur, Lister's techniques were adopted in Paris in 1873, six fears after he had published his findings in The Lancet and twenty-two years after the Parisian obstetricians had dismissed von Arneth's message. Pasteur finally completed the circle a decade later—nearly a half century after Semmelweis's discovery of attendant-borne transmission—by identifying the streptococcal bacterium that causes childbed fever.