A typical cold will last only about 10 days. Sometimes you can wait it out successfully or try different treatment methods until symptoms dissipate. Other times, it might feel like you’re in the grasps of a cold that just won’t let go. That pesky cold could be the flu.

Though they have similar symptoms, these illnesses aren’t the same. Knowing the difference can be vital to getting the proper treatment and preventing the flu from morphing into something more serious, like pneumonia. The elderly, young children, pregnant women, and those with weakened immune systems are especially susceptible to flu complications.

Understand how your immune system works; learn about the lifespan of the common cold; know when you should stay home sick—and when to seek emergency care—to protect yourself  and your loved ones against the cold and flu.