Yeah, the guys who gave him the antidote think that when they gave him the injection it didn't actually work because the virus had already mutated and that the reason he seemed better was because he was asymptomatic. Which explains his sudden turning after they thought they had already cured him. It's really sad, but I still have hope. I couldn't quite understand it much because I literally google translated the spanish version but it sounded like the seme's body is actually fighting off the virus and effectively killing it, which could also explain why he didn't turn for so long and why he came back to his senses after biting the uke. If that is the case then they could possibly come up with another antidote using his blood to save the uke. I wouldn't count on it tho, I'm hoping that's what they said but I could be wrong. Like I said, it was a little hard to understand since I just google translated it.
**SPOILERS FOR UP TO CHAPTER 9**
so they go meet up with the uke's friend, but that bastard was infected and turned on them. Then he took a giant ass bite of the seme and I wanted to just (╯°Д °)╯╧╧. Then by some fucking miracle the uke chanced upon some guys with an antidote and saved the seme. All seemed well and I was like "okay happy ending (/^▽^)/". BUT THEN the virus, that motherfucker (╬ ̄皿 ̄)凸, decided to MUTATE like it was the goddamn ninja turtles. Next thing I know, while the two were being loved dovey, like the cuties they are
O(≧∇≦)O The seme turns into a zombie and takes a mean ass chunk out of the uke's arm. And chapter 9 ends with the seme coming to his senses as the uke's lying on the floor with a hole in his arm. He calls out the uke's name mortified at what he has done, and BOOM end of chapter 9.