Seriously. If you can possibly afford insurance, then there's no excuse for not having it. I'm especially thinking of young people doing a startup and not buying insurance in order to "save money". That's a really dumb optimization, and suggests that you aren't very good at making trade-offs. A quick search shows that in California you can get a decent plan for $77/month. If you can't find $77/month, then maybe you aren't ready to do a startup.
I'm reminded of this issue right now because I just passed someone in SF laying face down on the pavement, not moving. His bicycle was nearby and I'm guessing that he had just been hit by a car. I hope that he does ok, but it didn't look good. (people were already helping him, and the police were just a few cars back, so I didn't stop)
Even if you are young and healthy, bad things can still happen. My brother was perfectly healthy until he got cancer at age 33. When my daughter was in the hospital, it cost $12,000/day -- she was there for almost 3 months.
The simple fact is that the health care system in the US isn't setup for people who don't have insurance. If something bad happens, you don't want to be one of those people. I'm not looking to debate our system, so please don't bother. Right now, the system is the way it is.
My point is simple: Do what you can to get insurance. Now.