system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when I'm sick my voice gets so much deeper?
gpt
As phlegm covers your vocal chords (or they get inflamed from coughing, etc) their resonant frequency is lower--like the thicker strings on a guitar have a lower pitch at he same tension--because they vibrate more slowly.

Since the vocal chords determine voice pitch, that pitch lowers.