For most of history, poliomyelitis was a relatively unremarkable disease – it caused paralysis and occasionally death, but only in a tiny fraction of those infected. It was essentially unknown in infants and adults, and usually only caused mild symptoms in children. This all changed in the early 1900s, when the disease mysteriously transformed into an epidemic, killing many and maiming many more, even among the supposedly 'protected' populations of adults and babies.
Deadly recurrences of polio became a fact of life in developed countries, particularly in cities during the summer. Movie theaters, beaches and swimming pools were closed; families fled to the countryside when the weather got warm. Clearly something had changed, but what could cause a mild disease to turn into a killer all but overnight? The secret lies, paradoxically, in our better understanding of sanitation.
In babies, polio can be mistaken for a mild cold – if there are symptoms at all – because they still have protective antibodies left over from their mothers. This early exposure was enough to make the infant immune to that particular serotype of the disease for the rest of his or her life. It's only when children grow older and lose those maternal antibodies that a polio infection can present in its devastating, paralytic form. Clearly there was something new to prevent the early exposure of infants to the polio virus. One major clue was the fact that the disease primarily affected white, wealthy families. The cleaner your surroundings were, the more likely you were to get the worst form of polio. Perhaps there was something in the water?
We now know that polio is spread through a fecal-oral contact route, and almost always through contaminated water. The adoption of modern plumbing, sewer systems and water treatment facilities in the late 1800s and early 1900s meant that infants were far less likely to be exposed to polio during the early 'safe' phase. Without that immunity gained in infancy, a chance infection later in life could be deadly. If your mother had herself never been exposed to polio, you didn't even have the blessing of a safe period in infancy. You, and your young immune system, were just as much at risk as older children and adults.
As with all new technologies, improved sanitation had some utterly unforeseeable ramifications. Clean water upset a millennia-old balance between poliomyelitis and our immune systems. Once one of the world's most feared diseases, however, polio is now all but nonexistent. After the epidemic peaked in the 1940s and 50s, polio went into a swift decline thanks to two successful vaccines. Keeping polio at bay, of course, depends on everyone getting their kids vaccinated – or going back to pre-modern standards of cleanliness.