Of Mice and Dust Bunnies

I keep a bottle/ container of hand sanitizer on my computer desk. I use it on my hands and every now and then I use it all over my computer mouse. If you look at the crud on the mouse, think of your hand there, creating that crud. Now, doesn’t it seem a good idea to keep both well cleaned?

How does crud build up like that? Has anyone got a really great theory? I expect it is the dust bunnies, as always. They are persistent little things.