It really depends on how you feel. If you have a fever, can't focus on your work duties, if you're coughing a lot, or if you just feel bad, stay home until you feel better. Since you may be contagious, your co-workers may actually thank you for it. If you do go to work, cover your coughs and sneezes with a tissue and wash your hands often. If you have the flu? Stay home. You probably won't feel like going in, and you definitely don't want to spread it!