I take sick days if I'm doing more than just the odd cough/nose-blowing/sneezing. Basically as soon as it hits "I'm not going to be able to walk down the hallway without blowing my nose" or "I'm coughing so much it hurts" or "I'm actually coughing stuff up... urgh" then I stay home. As I've done for the past four working days. I hope to be back at work tomorrow, though.