Grouch Flu is a disease that Grouches (and only Grouches) can get.

Its symptoms include optimism, politeness, affection, happiness, lack of "grouchiness", and just generalized "niceness".

Oscar and his niece Irvine and both got it in Episode 2630.

Later, in Episode 3589, Telly Monster worried that Oscar had caught it again when he was reading "nice books", but he didn't have it.

In Sesame Street Home Video Visits the Hospital, a young Grouch girl is in hospital with a disease, possibly Grouch Flu, whose symptoms include "feeling cheerful". When Big Bird recovers and leaves, so does the Grouch girl, who claims she is "feeling much worse".

The cure is rest, fluid consumption, and avoidance of contact with "nice people".

Oscar comes down with a similar illness in Episode 4828, dubbed as "kind-itis," a temporary sickness that makes him do acts of kindness.