system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When I get a bad cold, why does every muscle in my body ache?
gpt
Muscle ache (myalgias) and joint aches (arthalgias) are due to multiple factors, but they can both be traced back to the immune system doing its normal job.

Many chemicals are released anytime a cell is injured (from trauma or infection) or anytime a foreign looking protein (think bacteria & viruses & vaccines) gets into your body. Specific immune cells respond to those chemicals by (1) moving towards the source, and (2) releasing other chemicals (cytokines). This is the beginning of an inflammatory reaction.

Inflammation is simply a word describing the collection of effects that result from those chemicals. They include (a) redness (rubor), (b) swelling (tumor), (c) heat (calor), and (d) pain (dolor). (Yes. Some of those chemicals will actually cause you direct pain, so that's part of the answer.)

Now imagine you just stepped on a nail. Cells are damaged and an inflammatory reaction has been started. Some of the chemicals that are produced at the site of the injury/infection make the blood vessels near your foot get bigger and more leaky. A larger diameter blood vessel means more blood flow to the area, which means more blood cells are nearby. Leakier vessels means that more blood cells are able to leave the blood stream in this local area. This is helpful because you want certain immune cells to collect near the injury site and begin attacking any germs that may have been let in by the nail and to start repairing tissues.

Likewise, a bigger sized blood vessel with more blood flow and more red cells will look more red in color! More blood flow also means more heat! A leakier blood vessel doesn't just let out more cells, but it also lets out more water. Water leaving your blood stream ends up gathering in the surrounding tissues, leading to swelling. Swelling causes pressure and can cause some aches.

The example gave is a nail in the foot. How does that translate to having a cold? Well, the answer is that the immune system doesn't really discriminate and pretty much reacts the same way for anything  it thinks "isn't normal." Having a rhinovirus or the flu virus buys you the same basic inflammatory process. SO DOES GETTING A VACCINATION!
One type of chemical that's made is prostaglandin. Prostaglandin E2 makes you have a fever and directly causes pain. Non-steroidal anti-inflammatory drugs (NSAIDs), like ibuprofen and naproxen, block the production of these chemicals. That's why they are considered a fever and pain reliever.

**TL;DR** When your immune system is doing its job, inflammation helps get the proper immune cells to the invading pathogens, so it can identify it and make effective antibodies against it. The chemicals involved in the inflammatory process are pretty harsh and can have unwanted side effects on other parts of your body. This is necessary for acquiring "immunity." *People often mistake these annoying symptoms for "getting sick" after they receive a vaccination.*