eli5 Why do your joints ache when you have the flu?
When your body has an infection, the white blood cells need to gain access to the infected areas. Many white blood cells will be circulating in the blood and will need to leave the blood vessels to get into the tissues where the virus/pathogen is located. This is called extravasation.
The blood vessels themselves are made of cells that are lined up side by side. During extravasation, the white blood cells will send signals telling these vascular cells to stand apart (leave space between them) so that the WBCs can get through more easily.
This has the good effect that the WBCs can get where they need to go but the disadvantage is that fluid from the blood also leaks out.
This fluid can cause swelling and puts pressure on the tissues causing pain.
Tissues in the joints tend to inflame when you’re sick. Those inflamed tissues are now under more pressure than they should be, which causes the pain.
The inflammation is a side effect of proteins our white blood cells produce in order to fight infection. Our own tissue also reacts somewhat negatively to it.