I am a paramedic - the only things that will really do anything are gloves and mask for you (to protect against exposure) and some bandages so you can apply pressure to bleeding if you need it. Maybe a space blanket. Anything else will be pointless if you can't get the patient to definitive care, which is why the first step in any emergency algorithm is to call 911. An American Heart Association CPR/First aid course would do a layperson much more than any amount of equipment.
All I carry in my car are nitrile gloves, an equine first aid kit (better/bigger bandages than human stuff) and a fire extinguisher. The most important one is the fire extinguisher.