Before 1900, the polio virus was endemic -- most people got a mild case, easily mistaken for a bad cold, in their childhoods. After recovery, one was immune. A very few got the effects for which the disease became known in the 20th century -- atrophied or paralyzed muscles or death. (An ancient painting of an Egyptian priest from about 1500 B.C. depicts him with a withered leg and a foot held in a way recognizable in 1911 to a scientist as a polio effect; no one has challenged that retroactive diagnosis.) But it was the end of the 19th century before "infantile paralysis" was even given a name, and 1887 before the first of the epidemics that would mark the first half of the 20th century, this one in Stockholm.

The reason behind the upswing in the disease? Most likely sanitation, which prevented people from being exposed to so many germs and either dying of some other disease or gaining immunity. Polio tends to show up in developed countries; the longer one lived without being exposed to the virus, the more severe the consequences. During epidemics, mortality was lowest in children under five. When the virus stays in the intestinal tract, it does little damage, but in the estimated 3% to 5% of cases where it attacks the nervous system, the damage can be major.

The new name "poliomyelitis" was given in 1947 (by which time 25% of the U.S. cases were in adults), but the public quickly shortened the name. The 1940s and early 1950s were the worst years for epidemics of the disease, particularly the summers. It was never the biggest killer, but its unpredictablity and the grim images of crippled children and heads sticking out of iron lungs worried people far more than the actual risk of getting it. Unfortunately, stress makes one more vulnerable to the disease; many stories of the worst cases of polio involve an adult who thought they just had a mild bug and continued to work, pack for moving, or some other laborious endeavor until they were so sick that they could not go on.

For decades, no one knew how the virus spread; at different times the hypothesis was that it entered the body through the nose in breathing, through contaminated food, or through insect. The settled-on explanation is that the virus spreads through personal contact and usually enters the body through the mouth, but no one knew that during the epidemic years. Since it tended to be more common during summer, parents often kept their kids at home from public gatherings, especially pools, and sometimes limited them to their own yards; radio programs pitched in with trying to keep the children amused, but parents were often reduced to yelling, "Do you want to spend the rest of your life in an iron lung?" and such it's-for-your-own-good arguments. Since physical fatigue was known to make one more vulnerable, parents also might try to keep their child from playing too hard. But much misinformation circulated about what would or wouldn't protect you. The National Foundation for Infantile Paralysis in the U.S. tried to keep people calm about the disease, but weren't always successful.

Jonas Salk's invention of a polio vaccine caused cases in the Western world to drop off greatly in the late 1950s, but the disease continued to be a problem elsewhere, especially since the Sabin oral vaccine which was later adopted is heat-sensitive and can go bad in the tropics. In 1988, the World Health Organization called for polio's elimination throughout the world by 2000; this was not completely achieved, but there has not been a case in the Americas since 1991. As of 2001, south and central Asia and parts of Africa are the only parts of the world with wild polio virus.

Also, about 20% of the time, former sufferers of paralytic polio later have postpolio syndrome, or "acute paralytic poliomyelitis sequelae." This can involve fatigue, muscle weakness, joint pain, problems breathing, swallowing, or sleeping that occurs 25 to 35 years after an attack of paralytic polio. People who had regained the use of previously paralyzed muscles ended up losing their use again after decades. Researchers say that it doesn't seem to be related to the actual polio virus, but seems to be a kind of accelerated aging in people who had the worst forms of the disease. Research into the disease, much of which was cut off after vaccines were reached, may have to start up again to solve these problems.

Black, Kathryn. In The Shadow of Polio: A Personal and Social History. Reading, Massachusetts: Addison-Wesley, 1996.