Argument: Insurance companies screen out those that need health care the most
Paul Krugman, Robin Wells. "The Health Care Crisis and What to Do About It". New York Times Review of Books. Volume 53, Number 5 · March 23, 2006 - "the only way modern medical care can be made available to anyone other than the very rich is through health insurance. Yet it's very difficult for the private sector to provide such insurance, because health insurance suffers from a particularly acute case of a well-known economic problem known as adverse selection. Here's how it works: imagine an insurer who offered policies to anyone, with the annual premium set to cover the average person's health care expenses, plus the administrative costs of running the insurance company. Who would sign up? The answer, unfortunately, is that the insurer's customers wouldn't be a representative sample of the population. Healthy people, with little reason to expect high medical bills, would probably shun policies priced to reflect the average person's health costs. On the other hand, unhealthy people would find the policies very attractive.
You can see where this is going. The insurance company would quickly find that because its clientele was tilted toward those with high medical costs, its actual costs per customer were much higher than those of the average member of the population. So it would have to raise premiums to cover those higher costs. However, this would disproportionately drive off its healthier customers, leaving it with an even less healthy customer base, requiring a further rise in premiums, and so on.
Insurance companies deal with these problems, to some extent, by carefully screening applicants to identify those with a high risk of needing expensive treatment, and either rejecting such applicants or charging them higher premiums. But such screening is itself expensive. Furthermore, it tends to screen out exactly those who most need insurance."