You probably will need a vet. There are 3 causes for RI, bacterial, viral, fungal. So I would first make sure there is no mold in the substrate or anywhere in the cage. I would also raise the ambient temp and like ajmreptiles said, give it a week with higher temps and pray his own immune system clears it up. If it doesn't, then you will need a vet, so I would start looking now. As for why he got it, most of the common stuff is all around us, just our immune systems fight it off but if the immune systems get weakened or compromised, then the germ takes hold. Your snake is probably striking because he doesn't feel good I would guess. He's not striking because he's hungry. Unless you are wiping your hands all over a rat or mouse and then sticking your hand in the cage, he isn't going into feed mode. He's striking from like I said, discomfort or defensiveness.