If you're just sick with something minor, playing games isn't so bad and I'll do that quite often.
Now if I have something like a fever or severe nasal congestion, or something that gives you that horrible "heavy head" feeling... then I really can't play games. Usually your head ends up pounding because your body wants you to rest or sleep. Better in that case to drink as much fluid as humanely possible and rest.
Gamertag: Squ4rex "You guys can sig that if you want." -TheArcade