When you consider how controlled the operating room is today, it was just so wildly different before there was an understanding of germs. The surgeons, they wore their street clothes. They wore aprons that they never washed that they encrusted with blood.

Zhang: The bloody aprons were almost badges of honor.

Fitzharris: They were. There was one hospital that had a frock, an overcoat they hung in the operating theater and each surgeon would wear the same frock as a sort of a badge of honor, and it’s just encrusted with blood. Again, just no concept of germs or how unhygienic that would be. It was really just about the more that was encrusted on your apron, your frock, or your overcoat, the more seasoned you were as a surgeon.

And they never washed their instruments or their hands. The operating tables themselves were rarely washed down. These places became a sort of slow-moving execution for the patient because they would develop these postoperative infections that would kill them, sometimes within days, sometimes within months.

Zhang: A memorable phrase in your book comes from doctors praising the “laudable pus.” Why did 19th-century doctors believe that pus—which we now know is a sign of infection—was actually good?

Fitzharris: It really was because postoperative infections were so common. The rationale was it was somehow needed in order for the wound to heal. It was a good sign that wounds were suppurating.

Zhang: How does Lister start putting the pieces together on what is causing infections?

Fitzharris: There are doctors and surgeons who are starting to question the existing disease explanation, which is miasma theory—that is, disease is caused by bad smells. There’s discontent in this period. It’s a growing problem in hospitals. People are dying en masse. The solution that is thrown out there is that they should burn these hospitals down and start anew because the crisis is growing.

Ignaz Semmelweis in Austria noted that when doctors were going from the dead house to the dissection room and then birthing women in labor and delivery, they had higher frequencies of mortality rates, because, he reckoned, they were transferring something from the dead house to these women. But he still didn’t understand it was germs. That came later—that’s what Lister’s contribution is. He takes Louis Pasteur’s germ theory and he marries it to medical practice with antisepsis. And he is also the one who ultimately ends up convincing the medical community to adopt antisepsis.

Zhang: The antiseptic Lister ends up using is carbolic acid, which is extracted from coal tar, of all places. How did he think to use something like that on open wounds?

Fitzharris: He comes across an article in the newspaper that carbolic acid was being used in Carlisle [England] to kill the smell of the sewage in that area. He thought if it was good enough to kill the rotting smell of the sewage, then it might just do the trick on wounds.