a prion is an infectious entity and pathogen. proteins, as you may know (or not), are made of chains of amino-acids. composition dictates how these chains bend and curl, and form dictates function. now, a prion is a mal-formed protein, which causes other, similar proteins similarly denature: thus “reproducing”. they do this without DNA or other reproductive structures.
a common example of a prion infection is mad cow disease, and all prion diseases have similar prognosis: affecting the nervous system and being fatal.