Correlation without causation: The lesson of the Lazaretto

Correlation, of course, does not equal causation. But public health breakthroughs from the smallpox vaccine to the first smoking-and-cancer links came without it. So did the power of quarantine.

by David Barnes, associate professor in the Department of History and Sociology of Science, University of Pennsylvania

Published May 15, 2014, 6:30 a.m. ET

The pandemic of the moment is MERS: Middle East Respiratory Syndrome. This mysterious, untreatable illness—which kills nearly a third of those who catch it—

for the first time in late April. A

was confirmed this week in Florida.

And yet . . . after the new Lazaretto opened, the frequency and severity of yellow fever epidemics declined significantly. Quarantine shouldn't have worked—it was based on an erroneous understanding of the cause and transmission of yellow fever—but it did. Health officials in Philadelphia, New York, and other large American seaports saw cases of yellow fever every year at their quarantine stations, but only rarely saw significant outbreaks in their cities after about 1805. Every year without an epidemic vindicated the controversial practice of quarantine.

The Lazaretto Preservation Association of Tinicum Township is working to preserve the Lazaretto site for historical interpretation and public use. Contributions may be sent to LPATT, 629 N. Governor Printz Blvd., Essington Pa. 19029. The Friends of the Lazaretto invite one and all to a celebration of the Lazaretto's history on June 14 from 10 a.m. to 4 p.m. in Governor Printz Park, which is next to the Lazaretto on Second Street at Taylor Avenue in Essington.

Or take a video tour through history with Dr. David Barnes.