In a catastrophic event like a zombie apocalypse, first responders may be unavailable or overwhelmed. If trained first responders are unavailable in a medical crisis, you may need to become one.
When you arrive at a scene, make an assessment. Are bystanders rational or are they argumentative, screaming, or crying? Is the area safe?
The non-rational need to be moved away from the scene; they cause the patient stress, which could put them into shock. Rational bystanders can assist you if needed. Ask them if they have medical or first aid training, and utilize it.
Stay calm; do your best.
Source: Dave Terry, First Responder