Complex sentence: The CDC has cured Kellis-Amberlee, but doing so destroys the immune system, so instead they focused on engineering the virus to prevent natural immunities from developing, while taking measures to ensure they remained in power.
Split into simpler sentences: The CDC has cured Kellis-Amberlee, but doing so destroys the immune system. Instead, they focused on engineering the virus to prevent natural immunities from developing, while taking measures to ensure they remained in power.