it's just your immune system working...it feels 1mL of prop or sus or whatever sitting in your glute and it KNOWS it's not "self" so your body sends chemicals to go eat it...the fever is your body's way of trying to raise it's own temperature to a temp that would make you (as a host) uninhabitable to what it thinks is a viral/bacterial infection...I've never had it except recently (lethargic 8-12 hours post injection with steady 100.7 fever...WBC around 16,500 and monocytes through the roof)...everyone's reaction is different based on immune sensitivity...