Fun fact: I wrote a paper on the four horsemen and the horse which represented conquest is equally plausible to represent plague as well. Basically, I wrote; what attacks all nations and never ceases to stop? Plague. What becomes the king of conquerors when the conquerers can't stand against sickness? Plague. The bow and arrow of this horsemen represents the horseman "shooting" sicknesses and epidemics from a distance. This is also a symbol, for a bow can be used as a weapon that catches people off guard when they look down to see the arrow in their stomach. Now replace arrow in their stomach with symptoms of an epidemic, and both reactions would be equally surprised.