This means that you have been poisoned (by rattle-snakes, giant scorpions from Hell, or Iguana-on-a-Stick that has gone bad from sitting out in the sun too long.) Note that this is dangerous, but not necessarily fatal. Poison does damage over time. The more poison that you have been infected with, the longer you will take damage. Fortunately, as the poison starts to wear off, it takes longer and longer to damage you again.