Loser's Lurgy

Loser's Lurgy is an illness, probably not proven to exist, that apparently causes people to perform their activities poorly, i.e. to lose. Luna Lovegood is the only known person to believe in the existence of this illness, though her father probably believes in it as well. When she commentated on a Gryffindor-Hufflepuff Quidditch match in 1996, Luna speculated that Hufflepuff Chaser Zacharias Smith was suffering from Loser's Lurgy because he was performing poorly, to the immense amusement of those listening.

Behind the Scenes
Given that Luna and Xenophilius Lovegood tend to believe in many things that others do not, it is likely that this illness does not exist. This is supported by the fact that the word lurgy is British slang for "a fictitious, yet highly infectious disease", equivalent to the term "cooties".