As is the eventual fate of nearly every game, Red Dead Redemption's online multiplayer has succumbed to hackers. Many returning players are discovering this with sadness, now that the game is backwards compatible on the Xbox One.
However, this time it seems there's more to it. In an incredibly well put together post by Reddit user /u/InsaneTomato95, the various dangers you need to watch out for are discussed in detail.
The basic situation is that Rockstar has stopped supporting the online portion of the game, and it has literally become the wild west. Modders and hackers with jailbroken consoles have all but taken over. But the kicker is that there's a specific mod where players can ride zombies like a horse, but the mod is also partially a glitch, and without the mod it can cause issues in your game.
To a person without the mod, the zombies appear as an NPC glitched into the ground. These NPCs give unlimited XP when you kill them, because while you get the XP for the kill, it doesn't die and you can do it infinitely. Unfortunately, this is what causes the corruption on your game, which you can then spread to other lobbies and players.
The full Reddit post is here, and if you plan on foraying into the Red Dead multiplayer any time soon, you absolutely should read the whole thing to understand what you're going up against.