Dirty Focus Screen

I keep wondering how dust and other stuff keeps managing to show up in my view finder. Last week I was in the wilderness shooting stuff with my 50D and had a HUGE dust spec in my view finder. I had on my 24-70 2.8 on and didn't change lenses all day but had a huge mess in my view finder at the end of the day. I managed to blow a lot of it out when got home but had to remove my focus screen with a pair of tweezers and spend a lot of time using my Giotto Air Blower. How does this stuff manage to get inside my camera? It appears everything is pretty well sealed. I still have a piece of link or something in my view finder that won't come off. Haven't fingered out if it is on the focus screen or mirror. I hear to NEVER touch the mirror with anything so I haven't tried anything other than a blower to remove it.

Thanks