I have cyclothymia and panic attacks, and these issues were worst at 25-27. At that time, I worked for a startup and had no insurance. (Now I have shitty insurance, but that's a world better than none.) Of course, panic attacks are brilliant and creative in their ability to throw weird symptoms (phantom smells were my "favorite") and every time, I'd worry about the (small) chance that "this time" it was a physical health problem, which I couldn't afford to have. I still have a brutal (and, objectively, unreasonable) fear of hospitals. It's not so bad that I wouldn't go if I needed to, but even if it's to visit someone who's fine, it takes serious chemical assistance. I can take on a cross-country plane ride without klonopin (hell, most "normal" people in the professional world can't do that, these days; granted, that has more to do with yuppies being pussies than a real anxiety condition, though) and doctor's offices are fine, but hospitals... fuck. Too many horrible memories.

There are some great individual doctors and nurses, for sure, but our mean-spirited, psychotic medical/insurance system taps into this deep-seated fear that humans have of being abandoned. Abandonment is a more vicious fear than death, for most of us. Death (when it comes peacefully; granted, that's not as common as we'd like to believe) is either nonexistence or something that's probably very interesting.

To answer your question, you can go to the ER and get treatment without insurance, but that's not going to get you preventative heart surgery or cancer treatment. If you get cancer, you can't walk into an ER and demand chemo. For plenty of Americans with cancer, their first medical attention is when the fucking thing throws a clot or causes organ failure and it's past too late.

You won't go to prison for non-payment, but you will get a huge bill, because while insurance companies are good at negotiating medical bills, you can't. If you can't pay it (and U.S. medical bills can easily top $10,000 for a routine ER visit) then you can be taken to court, have a judgment against you, and end up with your credit destroyed (there's nothing on your credit report to say that the cause of your debt or bankruptcy is a black-swan medical event, or even medical at all) and your wages garnished.

It's not like these horror stories are common, but they certainly happen and most Americans know someone who's been involved in one. It's not like 25-year-olds dying on the street due to our bad medical system is a common event, but it is a terrible system that ruins lives on a fairly regular basis. The sword of Damocles hangs over all of us. Even with insurance, it's easy to get fucked. Michael Moore's Sicko (whatever you think of him, that documentary was incredibly well-researched and important) was about people who had insurance.