response: The cause isn't understood, but several different mechanisms have been proposed.
original_response: It can be biological or infections or genetic. Tests must be performed to find out the cause.
history: My friend has chronic fatigue syndrome.  Its sad That's too bad. I'm aware that this limits a person's ability to do daily activities. Yes.  He has a lot of trouble doing even everyday things in life
knowledge: Biological, genetic, infectious, and psychological mechanisms have been proposed, but the cause is not understood.