Just gonna chuck my two cents in here as an egotistical Halo lore maniac
As ReClaimer8015 pointed out, the Flood actively chooses its victims - the Forerunners during the 100,000 year war against the Flood spent hundreds of years experimenting not just on themselves, but on Humans to find a cure. There were reports that the Flood were avoiding a specific portion of Human populations to not infect them, and those who were not infected were eventually dissected by the Forerunners seeking a cure. The issue is that there never was none.
A flood infection permanently changes the architecture of DNA itself, as well as the nervous system and bodily structure of who it infects. In order to cure the flood infection, it would require an extreme level of manipulation on a genetic level in order to restructure all the damaged and re-worked strains of DNA. Not only that, but the permanent damage done to the nervous system and the structure of the infected body would be excruciatingly painful to rectify and return to a "normal state". In my opinion,
there is no cure for the infection. Once infected, there is two ways out - dissected until death, or killed.
HiddenXperia is one of my favourite Halo lore youtubers, he has a
full playlist full of information on the Flood and its history. I feel that if you do commit to this story (as all stories should be told, regardless of their issues), do take into account the entire history of the virus. No sugarcoating it, but don't go too far. A story you could tell (if you opted to discard your crazy scientist cure story) could be about a squad of Marines during the Flood invasion of Africa attempting to escape Voi while the Swords of Sanghelios glass the infection to save the planet. For that, take into reference the Halo: Evolutions story
"Mona Lisa".
Good luck, and have fun!