The no-talky parasite? I've been wondering if that's the Kojima take on secrecy in the armed forces. Many soldiers never say anything about their experiences, or not to those that really know them (their friends and family. ie: those that really speak their language). But not even to other soldiers. Even ones that were there, on the same side, right beside them.
The idea of the parasite infecting the next person represents the passing of a secret. Once they know it, they can't tell anyone either.
Sometimes there's good reasons for this. A soldier might have left the forces years ago, but the people they worked with might still be there. Or the next group of people. Any information or detail, no matter how small, might cost lives. And that would figuratively kill the soldiers that said anything, not knowing if they gave away information that harmed their friends.
Sometimes it's specific, about a certain country or events. Sometimes it's just a general blanket "don't talk about it" thing.
Of course, in Kojima-verse, it's literal death. And a bit weird, like always.
I liked how they did it though, with reference to the old WWII Navajo code talkers (that didn't come to light until decades later in some cases, but they did the right thing and never talked), but then also the "modern" parasite of enforced secrecy. Or rather, not enforced, but on pain of death. In Kokijima-verse, your own. But in real life, some ex-soldiers would rather die than talk about it, because they might be killing their mates if they do. They'd also never want to share the burden of the secrets they keep, because it would be even worse if their friends or family let slip the information that killed their mates, because they didn't know what could and couldn't be said, or what was important, yet the soldier would be the original cause of it.
Many things go to the grave. Often with good reasons. But it's hard on the soldiers themselves, and it changes them forever.
Even if there are good reasons for it, the no-talky thing is considered a problem for ex and current soldiers, being a contributing factor in post-traumatic stress, lifestyle issues, trust, depression, and even possible suicides. So it's one of the many themes in the game, pretty front-and-centre when you see it. With the overarching theme of loss and how different people deal with it as the "Phantom Pain" from the title. Snake's arm too, but that's not the focus, just a literal representation. Absolutely every character has lost something/someone in the game, sometimes even themselves or their dreams, and so I think that the Phantom Pain of that is the actual thing that was trying to be portrayed. The memory of what once was can hurt more than anything, but people all deal with it in different ways. Some good, some bad, but who are you to judge? You don't know how much the thing that is not there, hurts that person today.
There's always some whacky, yet very deep themes in MGS. Phantom Pain has tonnes of them if you look, and it's really well done.