It is the first stage of 5 stages in the fight against an invader.
Your blood vessels open, more blood is sent to the affected area and your white cell start to "argue " with the invader.Tempers rise and so does your temperature making you feel hot and this shakes the body into regulating this temperature rise. You sweat, shake, your skin pales and you feel drawn and "under the weather."
You then either win the battle, OR if the white cells lose the fight, then you go on to the second phase.
Normally you win the battle.