It's as though Dr. Kafka were put in charge of "General Hospital": If you've been sick and are likely to need medical care, the place you'd expect to turn to for help in paying the bills may not be there for you. That's because health insurance companies can't make what they consider a decent profit if they cover too many sick people. This may make perfect business sense to insurance underwriters, but it doesn't make much medical sense to patients.

The stumbling block in getting health coverage lies in three key words -- "pre-existing medical condition." What this innocent-sounding phrase means is that if you have had leukemia, epilepsy or severe heart disease, you may be medically uninsurable. A "Dear John" letter from your insurance carrier will inform you that you are denied coverage or that payments for the one disease you have will not be covered. Or, in a truly Kafkaesque twist, your premiums will go up so high that you can't afford the coverage. And without medical insurance, you may not get the treatment you need.

As the economy weakens and layoffs continue in private industry and local governments, more Americans are going to find themselves not just without a job, but perhaps without health insurance. If you can't continue coverage from your old employer, you may find yourself uninsurable because of a pre-existing condition.

The lucky ones who can ignore those three ominous words are federal workers and employees in large corporations. No one asks them if they have diabetes or if their child has asthma or even how old or fat they are. Instead, insurance companies and medical plans compete for their premium dollars because they belong to a large work force that can absorb the minority of cases that incur high medical costs.

Not so for small companies and people who are self-employed or unemployed. For them, getting coverage largely depends on their health status. They generally have to fill out a detailed questionnaire about themselves and their dependents. And the sicker they are, the more difficult and expensive it is to get coverage.

Take the case of young doctor Robert S. Mirsky of New York. He was setting up his private practice in the Bronx when he ran into Dr. Kafka. Mirsky had finished his hospital training and had temporary insurance under COBRA, the benefit named after the Comprehensive Omnibus Budget Reconciliation Act that extends coverage for 18 months to people who leave their jobs.

Then he discovered he had a myeloma, a rare bone marrow cancer. With his temporary coverage running out, he tried to get into another plan but was told in the insurance carrier's Dear John letter: "Your medical history is not in compliance with our current underwriting requirements." He found a policy for high-risk cases, but it wouldn't pay for cancer therapy for nearly a year. Mirsky managed to make a deal with the hospital where he sends his patients to pay for his care until the new policy kicks in.

As a physician, Mirsky knew who to call, how to work the system. But what about the rest of us? "I don't know how anybody gets through all of this," said Mirsky. "I was freaked out."

So was Cheryl Gresek of Ipswich, Mass., who gave birth to a son with a very rare heart defect. The first three months of his life cost $150,000. Gresek testified before Congress that her health insurance premiums were raised so high she could no longer afford the coverage. "Here we are, willing and able to pay a reasonable amount for insurance, and we're forced off," she said. "Our only fault is having had a sick child."

Just as hard hit as individuals are workers in small businesses with fewer than 25 employees. These businesses not only can't afford comprehensive coverage but often can't get it, in part because of escalating medical costs that have forced health plans to cut back on benefits while raising premiums and other out-of-pocket expenses.

Health insurers, in turn, say they can't always afford to write policies for small groups. "The risk of insuring a disproportionate share of high-risk individuals is much lower in the large group market, because those costs can be spread out over a large employee population," explained Diana C. Jost of the Blue Cross and Blue Shield Association at a congressional hearing in September. "Many insurers are now quite selective in the risks they will accept in the small group and individual market."

More than 30 million Americans have no health insurance at all. A 1990 consumer survey found that nearly 25 percent of people with chronic diseases such as multiple sclerosis, cerebral palsy and diabetes had been denied private insurance. The National Coalition for Cancer Survivorship estimates that one quarter of the 6 million Americans who have had cancer face discrimination in insurance coverage and on the job because of their illness.

All this adds up to national problem in need of emergency political treatment. Not just consumer groups but the health insurance industry, too, is calling for reforms and more government help in covering the poor and high-risk cases. On the bright side is the fact that most people -- nearly 65 percent of the population under age 65 -- have medical insurance through their employers.

Yet the number of medical have-nots is rising. And every time you apply to a new health plan, change jobs -- or your employer changes insurance carriers or your health plan goes out of business -- you run the risk of being penalized for having a medical condition.

Novelist Cindy Victor of Fullerton, Calif., put it this way after she was treated for breast cancer and then dropped from her insurance plan -- and found that she couldn't buy an individual policy because of her pre-existing condition: "I feel I represent Middle America -- I'm middle-aged and middle income. I pay the bulk of the taxes. But in my time of need, the government says: Up the kazoo, lady! Don't get sick until you're 65, or very very poor, or very very rich."