After completing the main mission ‘Aim True, Ye Vengeful’ in Metal Gear Solid V: The Phantom Pain, Big Boss will receive a codec call from Ocelot, informing him that there’s an infection spreading among the men on Mother Base. Though they’ve already set up a quarantine facility, there’s still the fear of the infection spreading elsewhere due to the lack of knowledge of what the symptoms are. The player will then be tasked with identifying which soldiers to quarantine.
First off, listen to the new cassette tapes you’ve obtained from completing the previous mission. Ocelot and Kaz will mention that the one thing the infected soldiers have in common has something to do with their mouths. Considering that language is a vital part of the game, it won’t be hard to come to the conclusion that the common language must have something to do with the infection.
Look through the staff members who are already in the quarantine facility and check what language they have in common. The infected language may differ from player to player (though Kikongo seems to be the most common option), but once you take a look at all the infected soldiers, just keep in mind that language is the common denominator and it should become clear which soldiers you should throw into the quarantine facility. For example, if Russian happens to be the one language they all have in common, then you should quarantine all the staff members who speak Russian.