Gearbox Software put out a Borderlands 2 patch for the Xbox 360 version of the game that eliminates the danger of data loss relating to the so-called "Graveyard virus," making it safe once again to play in public co-op sessions. Of course, "safe" is a relative term when you're talking about public co-op. Your save data might be safe, but any putz with a headset can still say awful things about your parentage, sexual orientation, race, religion, etc. and so forth. The fix for that, of course, is the Mute button.
The "Graveyard virus" emerged when the modding community discovered the remains of an unused difficulty setting that erases save data once a character dies for the first time. The spread of the "virus" appears to have been unintentional, with the tweaked difficulty on a modded save simply transferring over to any co-op players that joined in.That said, the very act of modifying a saved game is against the rules laid out in the Xbox Live Code of Conduct, so it's probably not something you should be doing.
Fortunately, you'll no longer have to worry about having your Borderlands 2 save indirectly modded thanks to the new patch from Gearbox. Note too that this only applies to Xbox 360 users; the PC and PlayStation 3 versions of the game were never impacted by the "Graveyard virus."