Pus

Pus is a yellowish fluid that is secreted in the human when they have an infection, such as pimples, as a defence mechanism against the infection. Magical animals can also secret such fluid, such as Bubotuber pus, which has high healing powers when diluted, but creates painful boils when undiluted.

The Fever Fudge developed by Fred and George Weasley causes large pus-filled boils to form in the eaters as a side effect on their rear bottom, but can be eliminated with a few drops of Murtlap Essence. Marietta Edgecombe has pustules in the formation of the word "SNEAK" for betraying the organisation, which activated Hermione Granger's jinx, and the pustules were immune to magical treatment, but eventually faded and left scars.