I have two male guinea pigs in a 5 x 2 c&c cage. Recently one of them has been nipping at the other but it has now turned into full on fighting. He runs and attacks his brother and the other one is completely terrified. It got so bad that the mean one drew blood and scratched him all over and took bites out before I could get him away. I separated them to keep the other one safe but the mean one is now biting my hand everytime I reach into its cage. When I try reintroducing him the same thing happens. I don't know what could be wrong or what else to do.
I'd keep an eye on the non-aggressive one. Guinea pigs can sense when something is wrong with another pig, and may annoy the sick one more than usual. But it usually involves following the other pig, mounting, etc., not outright aggression.