Spattergroit

"We're going to put out the story that I'm seriously ill with spattergroit, which is why I can’t go back to school. If anyone comes calling to investigate, Mum or Dad can show them the ghoul in my bed, covered in pustules. Spattergroit’s really contagious, so they’re not going to want to go near him."

- Ron Weasley

Spattergroit is a highly contagious wizarding disease. It may be a type of fungus, as Ron Weasley mentions that being unable to talk is a common effect of spattergroit “once the fungus has spread to your uvula”. A person sick with spattergroit will also get purple pustules over his or her skin and be bedridden, suggesting that fatigue is another symptom. It is potentially fatal.

To facilitate accompanying Harry Potter and Hermione Granger on the search for Horcruxes, Ron Weasley transfigured the family ghoul to resemble himself infected with spattergroit. Hence, when he did not appear at Hogwarts for his seventh year, his family could claim that he was too ill to attend.