In retaliation for the new Headmistress, Dolores Umbridge's tyrannical rule, the students used the Skiving Snackboxes sold by Fred and George Weasley to create temporary illnesses, with symptoms such as fainting, vomiting, developing dangerous fevers, or spurting blood from both nostrils. The illness can be traced back to Fred and George Weasley, the progenitors of the Snackboxes and who encouraged the students to rebel against Umbridge.
These massive waves of illnesses led Umbridge to scream and shriek with rage upon entering the classroom. Despite her efforts to trace down the source, the students stubbornly told her they were suffering from "Umbridge-itis". Taking it as an insult, she put four successive classes in detention. But after she failed to discover their secrets, the headmistress was forced to allow the students to leave in droves.
- Many Hogwarts students
"Itis" is a suffix originating from Greek meaning "inflammation of" an organ (i.e. bronchitis-- inflammation of the bronchial tubes.) It is also often used colloquially in the informal naming of illnesses, such as the so-called "senioritis" suffered by seniors in high school or college. In this case, the naming is particularly apt, as Umbridge became more and more inflamed and irritated with each instance of a student who feigned this illness in order to get out of one of her classes.