Which neurotransmitter(s is/are the body's natural pain killer?

Which type of nerve creates a 'mental map" of the relative positioning of the body at any given point in time?

An _ exposure is a repeated or prolonged exposure(over months or years) that may bring about slowly developing symptoms.

When children are young and go to the doctor, the doctor will give them booster shots. this re-exposes the children to a dead portion of a disease to them build immunity.