In the Comic Strip
During March 18-23, 1985, Garfield became diagnosed with the Hawaiian Cat Flu. The effects were that he starts acting like a pure Hawaiian and thinking that he's in Hawaii. Such as pretending to surf, wearing a flower shirt, dancing, etc.
In the TV Show
In the episode 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 secretly taking the medicine without telling Jon.
Alaskan Cat Flu
This is also a fictional disease that has the same effect as the Hawaiian Cat Flu that Garfield Now Has That He Thinks He's In Alaska in the TV show, except the name.