I believe that ADHD is just another name for right hemisphere brain damage. What causes right hemisphere brain damage? It can be caused by infections/illness, stroke, surgery, tumor or traumatic brain injury (TBI).
Now, with my dentist alerting me to the fact that Autistic and ADHD children are suffering from upper airway obstruction (lack of proper oxygen to the brain), Dr. Andrew Moulden discovering that these same children are suffering from microvascular strokes, and brain scans detecting lesions - what are you going to believe?
Everyday chemicals are thought to also weaken our blood-brain barrier too - making it much easier for chemicals and infections to enter the brain and cause neurodegenerative diseases!