Insurance companies use actuarial tables to determine how much to charge each of their customers, those more likely to become seriously ill (or already are) charged at a higher rate, so to force insurance companies to cover everybody with preexisting conditions certainly blows-up their business model, those companies unprofitable, doomed to failure, therefore, the people who otherwise wouldn’t be insured by one of these companies or would be charged a very high rate need be put in a charity pool (subsidized by private charitable donations for tax credits?), separate from the business model for the insurance companies, because by not setting it up this way would put the insurance companies out of business or on the government dole as part of a single payer system, socialized medicine.
