From cooking to showering, fresh coffee and laundry, we often take the availability of clean water for granted. Yet when Henrik Ibsen wrote An Enemy of the People in 1882, the specialty of bacteriology was in its infancy, and viruses had yet to be discovered, so scientists had just begun to understand how diseases could spread through contaminated water. Humans had tried for millennia to stanch the spread of contagions, but ignorance of these invisible-to-the-naked-eye life-forms prevented much progress. Nonetheless, many cultures developed strategies to purify their water, and we have savvy minds and societies to thank for the modern water safety enjoyed by most.

In ancient Egypt, shrewd citizens discovered that the chemical alum, when applied to water, caused the sediment to stick together and form clots large enough to remove. Quenching one’s thirst became a much more pleasant experience. A millennium or so later, the ancient Greek physician Hippocrates grew disenchanted with the grit in the water he used for medical treatments. He fashioned a crude system of filtering: pouring water through a cloth bag. This system came to be known as the Hippocrates Sleeve. He also advised boiling water before drinking, for flavor enhancement.

Until the 19th century, however, most efforts focused on removing visible contaminants from water; for much of human history, the notion of invisible contaminants would have seemed far-fetched. In both Europe and China, people subscribed to the miasma theory—or the idea that disease was spread through bad air. Those in Western cultures commonly considered night air as particularly dangerous, securing their doors and windows when evening fell; however, this caused disease to spread faster, as people spent nights in close quarters with others, including sick family and friends. As early as the 17th century, pioneering microbiologists such as Athanasius Kircher and Anthonie van Leeuwenhoek observed microorganisms under microscopes, and postulated they might be the cause of disease. Others dismissed their theories; the miasma theory held strong and infectious diseases ran rampant.

In 1854, London physician John Snow doubted that “bad air” was the cause of a deadly cholera outbreak. He traced the illness back to a water pump that pulled from a sewage-polluted section of the Thames River, and convinced city officials to disable the pump by removing its handle. On further investigation, Snow discovered that the pump’s well was adjacent to a cesspit in which a choleric baby’s diaper had been washed, and he suggested that the disease had spread via the fecal-oral route. This theory, however, proved too revolting to swallow for city officials, who replaced the pump’s handle as soon as the immediate crisis passed. The miasma theory still prevailed.

In the late 1850s and 1860s, French chemist Louis Pasteur made further advances in germ theory by discovering microorganisms originated from other microorganisms, and proving that microorganisms were the cause of a disease that affected silkworms—and in turn, the silk industry. German physicist and microbiologist Robert Koch elaborated on previous scientists’ work by creating a set of criteria for determining whether a particular microorganism caused a particular disease. “Koch’s postulates” stated that the microorganism: 1. must be found in all organisms suffering from the disease, but not in healthy organisms; 2. must be able to be isolated from a diseased organism and grown in culture; 3. should cause disease when introduced to a healthy organism; and 4. must be re-isolated from the inoculated, diseased experimental host, and be identified as being identical to the original specimen. Although these have since been amended (in particular, many organisms are exposed to disease but never become ill), they served as an important tool to isolate the cause of late 19th century bacterial illnesses. As more scientists and doctors accepted germ theory, they considered ways to apply their knowledge to prevent the spread of illness. English surgeon Joseph Lister used carbolic acid to sterilize surgical instruments and clean wounds, and his patients immediately suffered fewer post-surgical infections. Now considered a father of modern surgery (his memory is honored daily by garglers of his namesake, Listerine), Lister faced skepticism from many colleagues of his day.

As late as 1869, at the annual meeting of the British Medical Association, surgeon Thomas Nunneley charged that germ theory was incorrect, and therefore, Lister’s methods ineffective. By 1882, when Ibsen penned An Enemy of the People, germ theory had earned the acceptance of the scientific community with few remaining dissenters. But scientists, doctors, engineers and city planners  had not yet implemented their knowledge, nor had chlorinated drinking water or swimming pools become a solution. The widespread safety of tap water in developed countries would later rank among the most monumental human achievements of the 20th century. Yet, even as a man of the 19th century, Ibsen demonstrates a remarkable understanding of germ theory and bacteriology.

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