Winter is here—and with it, colds and flu run rampant. Everyone gets sick from time to time, and it can be difficult to decide when to see a doctor for a routine illness. If you head to urgent care and your illness turns out to be nothing more than a cold, you’ll be sent home with instructions to rest and take lots of fluids, and you’ll end up getting billed (and traveling to the doctor’s office) for nothing. However, some illnesses are more serious than others, especially if you’re older. While colds rarely develop into something more, the flu can strike (yes, even after a flu shot) and become very dangerous. You may want to seek medical attention as soon as possible if you think you have the flu. Here’s how to spot some differences between a cold and flu, and how to look for symptoms that should make you pick up the phone and call your physician.