Aresearcher isolates the blood of an individual who is sick. the blood is filtered through a porous material that will prevent all cells from passing through. the filtered serum of the blood is then injected into a healthy patient. this patient shows signs of the illness within days of the inoculation. what is the most likely causal agent of the disease based on this information? a) a virus b) a fungal pathogen c) a bacterial pathogen d) a protozoan parasite