system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that often when I'm sick, only one nostril is clogged or runny?
gpt
The body uses one nostril at a time when breathing. It shuts down the other nostril and conducts 'maintenance works'. Clears out debris and secretes mucus to help dislodge stuff into the back of your throat. 

When you are ill, you notice the congestion more due to the fact that ne nostril is already closed for maintenance while your 'breathing nostril' is having to breathe AND get rid off excess snot.