Look down at your body: are you bleeding anywhere right now? If not, maybe it's your head. Either way, bleeding is usually not a positive thing, but it's better to take preventative measures in the first place. To help, I've put together a list of 9 blood-loss diagnoses, and suggestions on how to avoid them. This list is posted on College Humor, but you should probably print it out and hang it up in areas where there's a high risk of zombies, time-travel, ghosts, or other bloody dangers.
NOTE: I am not a doctor, but I have watched lots of movies.
For more educational cartoons in the same vein, check out my Color-Coded Guide to Bomb Disposal and my list of Mirror Scare Variations.