A Stirge attacks and on hit it attaches to the body of the adventurer and start draining blood. This "attached" or "have attached" status is not a regular condition, but in the monster description is stated that, at the cost of an action, a creature or the subject itself can detach the Stirge.
So the question is: does a creature (or the adventurer itself) have some penalty to attack the Stirge? does such attack have a probability to hit the adventurer on a miss (like the optional friendly fire rule for ranged attack on cover)?
On a side note it seems pointless to spend an action to remove a Stirge while the creature has relatively low AC and 1-2 hit points.