In the Comic Strip
From March 18th to 23rd of 1985, Garfield gets diagnosed with Hawaiian Cat Flu after a rare strain of it is going around. Garfield's vet said that some of the symptoms of the Hawaiian Cat Flu were having a voracious appetite and being listless and cranky (which is how Garfield always acts). Garfield also started acting like her was in Hawaii--hula dancing, pretending to surf, wearing flowered shirts, etc.
In Garfield and Friends
In Garfield Goes Hawaiian, Garfield gets the disease. The effects however are different. Whenever he hears someone mentioning something Hawaiian he'll dance and thinks that he's in Hawaii. Jon signs him up for a TV show; however, Garfeld gets well after taking the medicine without telling Jon.
Alaskan Cat Flu
Another fictional disease--it's basically the same thing as the Hawaiian Cat Flu, except that the cat who has the disease thinks they're in Alaska rather than Hawaii.