if one have high fever, or serious injury, either go hospital emergency or your family doctor. Not every doctor will see you like Hong Kong, you just go any clinic and the doctor will see you, you have to have a family doctor first. That means - when you arrive USA, eventhough you are not ill or sick, you have to go find a famiy doctor and set up the records, otherwise, no doctor will see you when you are sick.
Anyway, you have to get a medical insurance, either government or private. The Obama Care - every one must have a medical insurance - otherwise you have to pay penalty for not having one.
I just consulted my mom. She said that once I reach 65 n get social security, u get part A of Medicare for free, which covers hospitalization. Yet, they will automatically deduct ~us$200 or so from ur social security checks for part b which covers out-patient.