Why was the symptoms so deadly 1348?

I assume that you are asking about the symptoms of the black death that ran rampant from 1348 to 1350 in parts of Europe. The disease started with a headache then chills and a fever. In certain cases there was Back pain, vomiting, soreness in the legs and arms and sensitivity to light. In a few days the victim would be experiencing painful sores that swelled and oozed blood and puss. There would be internal bleeding that formed dark spots on the skin that gave the black death it's name.