Answer:
There was an outburst of colds and coughs amongst the students that Madam Pomfrey was dealing with. She had a potion that that helped with colds but it left the drinkers ears steaming for hours and Ginny Weasley was one of those drinkers, and with her red hair it made it seem that her whole head was on fire.
(I tried! I hope you havea wonderful Christmas and I love Harry Potter!)