Danny has mitochondrial disease.
From Mito Action
“Mitochondrial disease is an inherited chronic illness that can be present at birth or develop later in life. It causes debilitating physical, developmental, and mental disabilities with symptoms including poor growth; loss of muscle coordination; muscle weakness and pain; seizures; vision and/or hearing loss; gastrointestinal issues; learning disabilities; and organ failure. About 1 in 2,000 people has Mito. It’s progressive and there is no cure.”
It’s not a “rare disease” on Marrs.