Price, Gaz, Soap, Ghost and you were on a mission to try to figure out the whereabouts of the nerve gas that Makarov had in his possession.
You were told that this gas was capable of tearing apart your own lungs, making you spit them out, including your blood before you asphyxiated to death in a span of 5 minutes. Obviously when you find the depot you find yourself trapped and Makarov's soldiers activate the gas before starting to shoot at you, unfortunately you were unlucky, your gas mask was broken due to bullet holes, the rest of the TF141 was successful to run away and hide, not you.
When the cloud vanished, Price, Gaz, Soap and Ghost noticed some dead Russian soldiers, he wondered how it was possible since they were not afraid to react, you suddenly emerge with a bloody mouth, spitting out a little blood and with red eyes. You looked like an undead.