Several years ago, before Obamacare went into effect, I heard an interview with a woman whose brother had a serious medical condition and whose medical insurance had been canceled, though he had faithfully paid every premium and done nothing wrong.
She looked into it and discovered that he had been persuaded to purchase a short-term health insurance policy and kept renewing it when the bills came, not realizing that he was, in fact, getting a brand new policy every time. When he got sick the company waited until the policy ran out and simply didn't let him buy a new one. Suddenly he had huge medical bills, no insurance, and no way to buy a different policy because of his now pre-existing condition.
Obamacare protected people from this. Insurance companies could no longer deny or cancel policies or sell cheap junk insurance like the kind this guy had bought. But now the Trump administration has issued rules again allowing the sale of the same crap short-term insurance.
In the four-minute video below pediatrician and medical school professor Dr Aaron Carroll explains why this is a problem: