Soon, Blue Cross coverage was available in almost every state, though not many people bought in. The modern system of getting benefits through a job required another catalyst: World War II. Thomasson says that if the Great Depression inadvertently inspired the spread of employer-based health insurance, World War II accidentally spread the idea everywhere.

"The war economy is an entirely different ballgame," Thomasson says. The government rationed goods even as factories ramped up production and needed to attract workers. Factory owners needed a way to lure employees. She explains that the owners turned to fringe benefits, offering more and more generous health plans.

The next big step in the evolution of health care was also an accident. In 1943, the Internal Revenue Service ruled that employer-based health care should be tax free. A second law, in 1954, made the tax advantages even more attractive.

Thomasson cites the huge impact of those measures on plan participation. "You start from 9 percent of the population in 1940 to 63 percent in 1953," she says. "Everybody starts getting in on it. It just grows by gangbusters. By the 1960s, 70 percent is covered by some kind of private, voluntary health insurance plan."

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