The Hawaiian Cat Flu is a disease Garfield catches on two different occasions.
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, causing Jon to say that they would never know if Garfield was sick or not. Garfield also started acting like he's 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 start doing the hula dance and doing some Hawaiian activities, thinking 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 disease, which is basically the same as the Hawaiian Cat Flu, except that the cat who has the disease thinks they're in Alaska rather than Hawaii.