Fleas

Fleas would be little black particles that fly around mobs, occasionally causing damage, which is annoying because it can interrupt tasks. As for how they are gotten, they may naturally appear on mobs, but certain mobs could also carry them around naturally, such as bats or rats if they are added. Pets who have fleas may get rid of them by going in water or using a brush to get them off The main reason I would add this would be to have a mob that acts like another suggestion I saw, which is Contagious Potions. Like those potions, fleas would be transferrable from mob to mob. They could be bottled and used to make the potions, although I am not sure as that may be too easily obtainable.