Drimwick's Miserable Missile functions exactly like Magic Missile, although it can be any level. If it is cast at higher than 1st level, it does an extra +1 damage (per bolt) for every spell level it is beyond first. You'll usually only see the 1st level version, however. When cast, it resembles a giant, rippling, green-and-black loogie that streaks towards its target.
If you are hit by a Miserable Missile spell, you take damage normally and one of your level 1 spell slots is forcibly converted into a Miserable Missile spell. This slot cannot be used to prepare anything else except Miserable Missile until the disease is cured.
At the end of any day in which you did not infect a new victim with frog pox, the frog pox spreads to another spell slot, starting with the lowest levels. At the end of any day in which you did infect a new victim with frog pox, you may choose a spell slot to return to normal.
The more slots that are filled with Drimwick's Miserable Missile, the froggier you look. If all of your spellslots are contaminated, you're sort of a sad, burbling, frog-man-thing. People call them "froglocks" and kill them on sight, lest they contaminate the village wizard with their froggy affliction.
Drimwick is widely regarded as the most idiotic wizard that ever lived. The function of this spell, if there is one, is a mystery.