Because humans have five senses: feel, see, taste, hear, and smell. When you step on a nail you basically going to feel it and you brain can sense that and it can trigger nerves which can cause pain.
This contamination now allow the virus to move into another living medium (the one who actually ate the potato).
After the virus enter the eater's body, it will start to reproduce itself and caused several symtoms during the period where the antbodies inside the host's body is fighting it.
