Hackney’s other rule of thumb is a little more complicated. He says it’s usually okay to go out for a jog if your cold symptoms are affecting you from the neck up. If you’ve got the sniffles or a sore throat, it shouldn't be a big deal to run. But if you’re coughing a lot and you can tell the cold has spread into your chest — say, you’re hacking up phlegm — it’s generally better to rest up before you hit the pavement. “If you’re running and you’re congested in your chest, it’s harder to get air in and out,” Hackney says. “You run the risk of it being more difficult to get oxygen into your blood. This is going to mean you’ll have to work a little bit harder when you run, which will make you a little more fatigued.” If you’re fatigued, it’s harder for your body to fight off the cold.