Your eyes are watery, your throat is sore, and your nose is running like a faucet. Yes, you've found yourself in the clutches of yet another cold, with little respite in sight. According to the Centers for Disease Control and Prevention (CDC), the average adult gets between two and three colds a year, the bulk of which last seven to 10 days. But what happens when those pesky cold symptoms don't subside in a week or two—or are followed by unfamiliar ones that seem more insidious? With the help of doctors, we've rounded up the cold symptoms that could mean a more serious illness is afoot.