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".
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".