Hi
As we are well aware with two great properties that are present in both viruses and prions, i.e Ability to reproduce.
Prions are basically very tiny little organisms which are considered to be even smaller than viruses, their structure consists of proteins in folded form and are capable to induce abnormal folding of proteins in certain vital proteins found in Human and some other animal brains, therefore when the enter in brain organ they multiply there rapidly and alter the function of proteins by altering their folding patterns and thus confer harm to the host.
If we talk about viruses they are non living entities outside the living host but once they enter living host they get an ability to live and reproduce. When they reproduce very abruptly in host organs or tissues, they often take the control of host cells and force them to produce materials for their own longevity and life, and in this way alter the host metabolism and organ functionality.
Hope it helps!