Answer: Penicillin

Penicillin was discovered in 1928 by Alexander Fleming, but it wasn’t until 1938 that another team of researchers isolated the antibiotic in a manner where it could be readily used to treat patients. Problematically, World War II era restrictions on manufacturing and production made it very difficult to produce penicillin in sufficient quantities.

Today we have the process down pat, but in the early days of penicillin production it took an astounding 2,000 liters of mold culture fluid to produce enough pure penicillin to treat a single patient. In order to meet the increasing demand for the drug, doctors used a clever trick. The majority of penicillin, anywhere between 40-99 percent, passes through a patient unused and is excreted in the urine; the penicillin that is not used by the body fighting the infection can be reclaimed by crystallizing the urine of a treated patient and extracting it.

In busy hospitals in the 1940s, before penicillin production could meet the demand, it was quite possible that the penicillin you were treated with had just been inside your neighbor across the ward.