Who is responsible for paying medical bills?

You sort of answered your own question, though you are making an assumption in doing so: you refer to "the person/business responsible" for the injury. If there is a party who is responsible for the injury, then that party ultimately must pay (and might have insurance to cover that liability) but first it has to be proven that the party is in fact responsible, and that question brings up a whole raft of issues. Meanwhile, the injured party has had medical treatment which needs to be paid for. If that person has insurance coverage, then the insurance company is obligated to pay according to the terms of the policy. Depending on the circumstances and the jurisdiction, the insurance company may have an opportunity to get its money back from the "responsible" party. Also, some businesses have what is called a medical payments provision in a general comprehensive liability insurance policy, called "med pay" for short. Under that provision, the property owner's insurance company will pay the medical bills of a person injured on the insured's property (up to a certain limit), whether the property owner is legally liable or not. But that coverage is not required. (If you are talking about a car crash, that's no-fault and that's a whole different ball game) If the injured party has no insurance and the property owner does not have med-pay, then the injured party is obligated to pay, and if there is any recovery from the "responsible" party, then those proceeds can be used to cover the medical bills or reimburse the injured party for the loss.