What causes transient flu-like symptoms rather than flu itself? an infection in body?

So many things: There are literally hundreds of possibilities, if you have fever that persists despite antipyretics such as tylenol, (acetaminophen) or additional symptoms other than flulike illness, please seek the attention of a physician for a personal evaluation.

Mainly immune system: Though direct viral killing can occur, causing some of the sx, in response to infections from viruses, your body produces a lot of different proteins to help signal and coordinate your immune system to fight infection, and to create a hostile environment to the virus. Examples includes interferon alpha/beta- this group of proteins can cause muscle aches, joint pain, and fever. Hope this helps.