A pathogen is defined as a disease-causing agent (line 2), hence assuming that the prion theory of infection is correct, we can infer that prion is a pathogen (line 24). Prions lack nucleic acid and consist mainly of protein (lines 22-24). Therefore, it must be false that nothing that lacks nucleic acid is a pathogen as (A) suggests since prions lack nucleic acid and are pathogens.
(E) is a true statement because the passage tells us that prions are protein pathogens, hence there are other pathogens besides viruses, bacteria, fungi, and parasites.