Virus

Your blood cells turn into diseased cells that you can use to harm your enemy.  When an enemy hits you, you may use a reaction to take a few drops of your own blood and throw it on the enemy that just hit you.  Your blood gives that enemy a virus that does not stop affecting them until it is removed or the enemy is dead. If the enemy dies with the virus any physical contact with the body will spread the virus.  While affected by the virus, 1d4 damage is taken at the beginning of every turn including others turns.