The death rates in American cities fell by 30 percent from 1900 to 1936. Assuring a constant supply of clean water was responsible for 13 percent — almost half the total reduction in mortality.

Pietro may not know the numbers but he knows their message. That’s why he feels proprietary about the water fountain in front of the medical tent at Occupy D.C. in McPherson Square and is so worried about what will happen when the city turns it off.

“To structure a community such as this to be more sanitary does a lot to offset being in a place where illness can spread pretty easily,” said the thin, black-haired 24-year-old, who has been a camp medic at Occupy D.C. since Oct. 24.

“Pietro” is his nom de clinique; he’d rather not give his real one. Until a few weeks ago, he was a senior sociology major at the University of Southern Maine. He quit two part-time jobs, moved out of his apartment and left school to come here — what he calls “a necessary sacrifice because this is the beginning of a movement and it’s important to be part of it.”

He sleeps behind the medical tent, which is the only yellow one in the encampment and its visual center. Red crosses fashioned from packing tape emblazon both the tent and Pietro’s blue pullover. Inside are two chairs, a cot and drawers of donated supplies stacked roof-high.

Pietro is certified in CPR and automated-defibrillator use, and he took a semester-long course in wilderness first aid in college. But he makes no bones about the limits of his practice.

He prescribes nothing but offers over-the-counter remedies — vitamin C, zinc and echinancea for colds, triple antibiotic cream for cuts — from plastic trays on a table. He has put on bandages, but that’s it for procedures. He has called 911 once, for someone who fell off a bench. As with medicine through the ages, attention and reassurance are his most important therapies.

On Wednesdays, volunteer physicians and nurses staff an evening clinic. With flu season approaching, would Pietro consider asking the city to conduct an immunization clinic at Occupy D.C.?

“I personally am opposed to vaccination,” he said. “But if we collectively decide to have that as one of our tools of the trade, then so be it.”

Over the past week he has been “on a mission” to make the fountain in front of the tent kitchen-use only, with the one on the other side of the park for drinking. But it hasn’t really worked. Now he and his colleagues are trying to find ways to get water — possibly in refillable jugs or a “water buffalo” tank — when the fountains are turned off for the winter. At least the site now has two portable toilets (paid for by the Service Employees International Union and emptied three times a week) and an arrangement that makes showers available at the AFL-CIO’s 16th Street headquarters for two hours each weekday.

Morale, Pietro says, is key to the body and the body politic.

“If you are feeling down about a situation, I guarantee you’ll be sick with something in 48 hours.”