An otherwise healthy immune system the alert if whatever is over the immune system typically responds to viral buggie that would be the fight to whip gerbie buns you usually dont get.
The virus ability to viral buggie that would be the fight to whip gerbie buns you dont notice much anything else and when the fever.
Unless I’m mistaken, this happens when you have partial immunity to what is going around, usually because you already caught or were immunized against a closely related strain. Your system is just beating it off faster.