From the Super Mario Wiki
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Roseitis is a strange type of disease, caused by eating a rose, that makes a person's ears to grow roses for at least a week.
In The Super Mario Bros. Super Show! episode "Flower Power", Luigi, wanting to cure his Veggieitis, which caused vegetables to grow from his body, is advised to eat a rose by Young McDonald to cure himself. Though eating this rose cures Luigi of his Veggieitis and causes all the vegetables on his body to fall off, it also causes Luigi to gain Roseitis, much to his annoyance and Mario and Young McDonald's amusement.