"Are you pondering what I'm pondering Pinky?"
"Wuhhh... I think so, Brain, but if a ham can operate a radio, why can't a pig set a VCR?"
When you enter the dungeon (which I typed as fungeon 3 times!) Battle effects happen first, which would discard the diseases. Following this the attack is compared to the hp of the monster (with light penalties). So no, the disease would have no effect.
I think you mean Dungeon Effects MAY be used first...not Battle Effects.
Regardless, the disease is gone from your hand at the time of combat...so the disease has no effect as Matt stated.