Got a bill in the mail under $500, if I don't pay it what happens?

The bill is a charge for a procedure I had done many months ago. I'd rather not pay an unexpected medical charge given that I don't know if I can. I have paid surprise, unfairly large bills in the past and I am tired of being taken advantage of.

I know I had not paid a bill in the past, it went to collections who contacted me once and then never again. What would happen if I don't pay this one? I think I can afford to pay it, it just would make things very tight. There's other surprises in life, like new tires, which might come up and I'd rather have the money for that then for a bill almost a year past the procedure date.