In 1888, a thirty-three-year-old MIT professor poured a glass of water, held it up for a class of young civil engineers to see, and over the course of an hour struck terror in the hearts of his listeners.

“He would scare us to death by saying that [the water] contained enough germs of typhoid fever to give the disease to a thousand people…,” wrote former student George C. Whipple. The professor then eased tensions by showing how engineering methods could be used to make water safe to drink.

For William Thompson Sedgwick—the passionate MIT biology department head better known as “The Chief” to his students—the science of tracking and eradicating waterborne bacteria was literally life and death. While the field of epidemiology was then thought of as belonging strictly to chemical and medical scientists, Sedgwick was one of the first Americans to suggest that “sanitation belongs in schools of engineering.” Just one year after frightening budding engineers with his water of doom, Sedgwick had the perfect research opportunity to show the world exactly what he meant.

In the late 1800s, Lawrence, Massachusetts, was (and to some extent still is) a small town run by European immigrants. Just six square miles in size, the town attracted flocks of Germans, French-Canadians, and Irish thanks to its burgeoning textile mills, cotton manufacturing plants, and scenic location along the Merrimack River. In the 1870s, the Essex Company manufacturing shop, in conjunction with the Massachusetts Board of Health, set up a small water and sewage treatment facility to prevent solid industrial waste from leaking into the town’s water supply. When a severe typhoid epidemic broke out in the town a decade or so later, the facility was the perfect place for a team of scientists helmed by Sedgwick to prove that disease can travel by water and that a medical degree wasn’t required to stop it. It became the Lawrence Experiment Station.

Before addressing those structural concerns, however, Sedgwick’s team first needed a way to calculate how prevalent waterborne organisms were in the Merrimack. The first step was to figure out a better method of counting the cells that live in liquid. Prior to 1889, scientists relied on the “cloth method”—a system in which infected water was literally filtered through a cloth to catch organisms living on the surface. That evolved into the conceptually similar “sand method,” which substituted fine-grained sand for cloth, but neither were particularly effective at helping scientists get accurate cell counts.

One year after beginning work at the experiment station, Sedgwick partnered with civil engineer George W. Rafter and pioneered the Sedgwick-Rafter Method, which used a sand filtration system to isolate water-dwelling organisms and a specially-marked glass disk placed on top of the microscope’s eyepiece to make cell-counting easier. Better known as an ocular micrometer, this disk was engraved with a small, square grid which created a manageable area for counting cells and an easy way for researchers to extrapolate the number of cells in a given sample.

With the new method in place, Sedgwick’s researchers analyzed water samples from across Massachusetts, eventually tracking typhoid outbreaks in Lawrence and nearby Lowell to river pollution. “The proportion of sewage that has been directly mingled with the water of the Merrimack River is, at the time when it arrives at Lowell, about one part of sewage in every 1,200 parts of water,” Sedgwick wrote in an 1891 report to the Water Board of Lowell. “That is equivalent to about a thimbleful of sewage in every quart of city water.”

To prove that the outbreak could be stopped through engineering, rather than medical treatment, Sedgwick’s team encouraged Lawrence legislators to build a citywide water filtration system, the first of its kind in the nation. Implemented in 1893 despite significant legislative pushback, the filtration system reduced instances of typhoid fever by nearly fifty percent, according the Journal of the American Medical Association, and established Lawrence as the first town in the United States to filter water for disease prevention.

The success at Lawrence not only set water filtration standards in locales across the globe, it also laid the foundation for a brand new field of study that examined health issues through a strategic planning and engineering lens. Sedgwick spent the rest of his life expanding this new field, in no small part through his work co-founding what is now the Harvard School of Public Health, an institution launched in partnership with his once terrified student, George C. Whipple. One-hundred twenty-six years after one professor scared students with a simple glass of water, modern day MIT profs will need something new to strike fear in our hearts.