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Sometimes, before it’s obvious that you’re sick with the flu (or some other illness), you can be pretty contagious, Daniel Pastula, MD, MHS, a professor of infectious diseases, epidemiology, and neurology at the University of Colorado Anschutz Medical Campus, tells SELF. Those early warning signs are your body telling you it’s time to rest up so your immune system can work more efficiently. If you pick up on those clues quickly enough, you could potentially start treatment sooner and recover faster.