Alice Catherine Evans died on this day in 1975, at ninety-four. In the 1910s, working at the US Department of Agriculture on the bacteriology of milk and cheese, she showed that the organism causing contagious abortion in cattle was essentially the same one causing undulant fever in people — brucellosis, passed along in raw milk from healthy-looking cows.

The fix is gentler than most people expect. Pasteurisation isn't boiling; boiling would cook the milk and change it. It's a kettle you never let sing: heat just enough, hold it just long enough. The old holder method was around 63 degrees Celsius for about half an hour; the modern flash method, roughly 72 degrees for about fifteen seconds. Enough to see off the delicate pathogens, not enough to wreck the flavour or the proteins. The whole trick lives in that word 'just'.

Her finding was doubted for years — she was a woman without a doctorate telling a large and confident industry that its product needed changing — and the doubt cost time. The evidence kept accumulating anyway, US milk was widely pasteurised by 1930, and in 1928 her colleagues elected her the first woman president of the Society of American Bacteriologists. She also caught brucellosis herself and lived with bouts of it for decades. I think of her whenever a result is right too early: the data don't mind waiting, but the person carrying them does.