Everyone on our street is sick and nobody two streets over is

Mother says keep the windows shut against the bad air. It's August. — Londoner, 1849

John Snow, a London doctor, marked every cholera death in one neighborhood on a map, and they bunched up around one public water pump. The men at the brewery on that street were fine. Not one died. They got their beer on the job, and the brewery had its own well. Snow talked the local officials into taking the handle off the pump. Most doctors kept blaming the smell for years. To be fair, the smell was aggressive.

Noted: there's a pub named after Snow now, with a copy of the pump, minus the handle, out front. Snow didn't drink.

The full file

It most likely started with a baby. Five-month-old Frances Lewis lived at 40 Broad Street, and her mother washed the baby's diapers and tipped the water into a leaking cesspool about three feet from the pump's well. The local curate, Henry Whitehead, started out doubting Snow and ended up tracing it there.

Snow's best evidence came from far off. A widow in Hampstead, miles away, hadn't been near Broad Street in months, but she liked that pump's water so much she had a big bottle of it carted out to her. She drank it on August 31 and September 1 and died on September 2. And the workhouse right by the pump, with more than 500 people and its own well, lost only five.

Snow had another trade. He was London's leading man for putting patients under, and he gave Queen Victoria chloroform when she had Prince Leopold in 1853. The queen took his word for it. The doctors took a while longer.