This event opens up the last third of the judgments of Revelation and the worst of all that have so far occurred, although not the worst one as there's still others after this.  This judgment falls only on those who have taken the Mark of the Beast and worshiped the image setup by the AntiChrist.  This event will likely begin after the desecration of the temple by him.  Those that are still alive, likely less than half the total world population at the time of the rapture, will be given two choices.  Either take the mark of the beast and swear allegiance to the AntiChrist, or die by beheading.

Those who gladly take the mark will immediately be struck down by this plague.  It won't kill them, but it's gonna hurt so bad they'll wish it had.  It's believed that this judgment will provide those who are unsaved one final chance to choose between Christ and the AntiChrist.  Take the mark and live temporarily, but ultimately die and spend eternity in Hell, or refuse the mark and risk being beheaded.  Refusing the mark won't provide instant salvation, but it will avoid someone being cursed and thrown into Hell because they did.  The person still has to choose Christ after this, as failing to do so will still result in Hell.  Just not the same level of torment as someone who took the mark.

This event will spawn the final major soul harvest on the planet during the Tribulation period.  Any further surges of salvation after this will be small pockets here and there of holdouts that didn't choose before or during this event.  As for where the sores come from, some have theorized that it's a result of the implant they will likely receive as part of the mark that will allow them to buy and sell.  However, more realistically this judgment will be entirely of divine origins as a punishment for taking the mark, as previously stated.