Paranoia and parasitic infections. 

Question the validity of this statement.

Other common paranoid delusions include the belief that the person has an imaginary disease or parasitic infection (delusional parasitosis); that the person is on a special quest or has been chosen by God; that the person has had thoughts inserted or removed from conscious thought; or that the person's actions are being controlled by an external force.

My research, suggests that infections, including parasitic ones can result in symptoms of paranoia.  The bodies reaction to a real threat of attack on the micro level.

Look up organic infections of paranoia.

Then there is the case of prescription drugs that cause this symptom...

 Levaquin side effects