There's no hard and fast rule about when to call in sick. Use your common sense and assess how you feel. If you can't focus, you have a hacking cough, or you believe you are contagious, it may be best for you, and your colleagues, if you call in sick. If you must go to work, keep your germs to yourself. Cover your mouth and nose while coughing and sneezing, wash your hands and use hand sanitiser often. If you have the flu, stay at home. You probably won't feel up to going in anyway -- and no-one will thank you for passing it around.