It's shock. You get it by taking a large amount of damage in a small time period (Falling, getting shot, many zombies killing you at once, etc.) If you log-out with this effect on, you'll be unconscious for the next 5 minutes next time log in.
What keeps on killing you? If it's zombies, stay away from the cities until you get a hatchet. If it's bandits, go to unpopulated servers and scavenge in non-dangerous areas. Teaming up with people is a great choice as well.