actually I have mysophobia and touching is generally one of the worst, especially around the face area
i think everyone is a bit different. in things that they can and can't handle, even if you have the same thing. and yes, they already said it was partially from trauma. It's the same as anxiety for shirotani. in the sense, that it starts with one thing. Him not wanting to be pat on the head or touched. to not wanting to touch anything. to thinking of himself as sick and able to spread that to others.
Everything he felt, was close to mystophobics, and that's what he decided to call him condition. though, over time, he subconsciously did develop the symptoms, making him one.
It's really obvious that he has mysophobia from trauma. Mysophobics normally aren't so dissgusted from touching. But things like door handles are full of germes, but he wrote it like no. 1.