It's often described as the former, but in practice is about the latter -- in particular, about being contaminated by germs, to the point of being irrationally avoidant of anything dirty or unclean. It was initially coined in 1879 by a doctor when describing a case of what today would be called obsessive-compulsive disorder, but while the two are sometimes found together they are distinct: you can be phobic without being OCD, and verse-visa. The roots coined from are Greek mýsos, uncleanness, filth + phobos, fear.
We have to cater to my mother's mysophobia and scrub the place top to bottom for a week before she visits, and she STILL finds something to clean.
Administrivia: Next Thursday I'll be traveling out of reach of any internet. Can anyone take over for me?