Brain fog can be caused by a number of things. I have Lyme Disease and MAJOR brain fog. I'm in long term treatment (both traditional and homeopathic) because I've had this disesase for many many years. I definately have spirochettes in my brain. (BTW - I get Lyme die off from Humaworm - it helps kill my Lyme, too)
Then I discovered when I started Chiropratic therapy that due to a 25 year old auto accident the disks in my neck were compressed and causing lack of proper blood flow to the brain. Yet another cause of brain fog.
I would think that some parasites would cause this symptom, as well.