After mission 25 you will get a message that your staff has been infected and you have to pick members of your team to put into quarantine. This will prevent the sickness from spreading and you avoid losing a lot of people.
In order to do this you need to identify the infected. Go into the menu and pick staff management. Eight people have already been taken to the quarantine for you. These eight all share one information that links them together. They all speak the language “Kikongo”.
What you need to now is go to “All staff” and find everyone in your team that speaks “Kikongo” and put them into quarantine. Depending on how many people you have working on mother base, this can take a bit of time.
The easiest way to find them select the ones you find in “All staff” (X on Xbox and Square on PS 4) and when you’re done move them all to the quarantine. You will get a message that the outbreak has been drastically reduced already.