system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how can you go to bed perfectly fine one night and then wake up the following morning sick as all hell with a flu or cold?
gpt
Believe it or not, the symptoms of flu or cold are actually the body's signs that it is finally reacting to the pathogen. With a flu or cold, the disease is actually active in your body for 24 to 72 hours *before* you start feeling symptoms. Yes, this means that you were contagious then, too. And it means that when you start feeling crappy on Wednesday, it's because you caught something on that date on the previous Friday night.