- "Meanwhile it became clear just how many Skiving Snackboxes Fred and George had managed to sell before leaving Hogwarts. Umbridge only had to enter her classroom for the students assembled there to faint, vomit, develop dangerous fevers, or else spout blood from both nostrils. Shrieking with rage and frustration she attempted to trace the mysterious symptoms to their source, but the students told her stubbornly they were suffering “Umbridge-itis.” After putting four successive classes in detention and failing to discover their secret she was forced to give up and allow the bleeding, swooning, sweating, and vomiting students to leave her classes in droves."
- — Hogwarts students rebelling against Umbridge's reign at the school[src]
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.
"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.
- Harry Potter and the Order of the Phoenix (First appearance)