system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do your muscles get so sore when you have the flu??
gpt
One task of white blood cells is to repair and rebuild muscle fibres which are damaged from day to day. However, when you have the flu, your immune system diverts the white blood cells from this function to fight off the flu virus, thus leaving your muscles and joints feeling sore and achy.

In addition, white blood cells produce a chemical called cytokines when fighting the infection. This causes inflammation in muscles and joints, resulting in pain.