If you bought the rental car insurance, file against that. If not, I think you are screwed if the driver does not have any insurance on his/her own car that transfers over unless they have a credit card that provides coverage or if this was a corporate rental and the corporation provides coverage. You can try to sue the at-fault driver but you will need to keep in mind that you will be trying to collect with whatever his/her personal assets are only and if the person doesn't have a lot of assets or works for a lower wage you might not get much very fast.
If you are an injured passenger and the at fault driver has no assets or insurance to pay your bills, here are some other options:
1) If you have health insurance, file with your health insurance company. They might cover you in this case.
2) If you have a car an insurance, file with your own uninsured motorist policy. They may cover you in this case. They should then go after the uninsured driver.
3) If you can pay in full for your own care, this is expensive but an option available.
4) If you are low income, see if you qualify for reduced cost care or charity care at the hospital you are in.
5) The hospital may offer an uninsured patient a payment plan in cases like this.
6) The other option is if you are willing to wait a little bit see if you qualify for a low cost or community clinic that caters to low income people will help you with your medical needs.
7) The last option is to have the hospital treat you and then file bankruptcy. This destroys your credit but at least you will be treated.
