This is one time when it's not a good idea to hit up a bootcamp class. "When you're sick, your body needs a significant amount of energy to fight the infection," Dr. Greenblatt told POPSUGAR. "Running the inflammatory response that you need to fight a major virus infection, like influenza, requires a lot of calories." She went on to explain that you can still exercise if it helps you feel healthy. "Just take a walk instead of going for a run," she said. "Don't make your body fight for those resources it needs to fight the virus."