Mexicanism
Is a highly contagious disease, those who suffer from it will eventually become Mexican. The symptoms include a browning or tanning of the skin, attempts at speaking Spanish, growing a mustache, delusions of being President of Mexico, and/or throwing up at hearing anything that is not Latin music.
Since Keith started dating a Mexican he began suffering from Mexicanism. Because of the disease he attempts to speak Spanish, stands out in the sun for prolonged periods of time, and listens to Latin Music on his Zune.