Nthe search to discover the agents that cause mad cow disease, scrapie in sheep, and cjd and kuru in humans, diseased brain tissues were passed through a fine filter to remove bacteria. the filtrate was still infectious, indicating that something smaller than bacteria, either viruses or organic molecules, must be the causative agent. if a virus was responsible for these brain diseases, then the infectious agent would contain either rna or dna. other possibilities were that the agent was a carbohydrate, fat, or protein. tissue filtrates were treated with agents that destroyed just one of these chemicals and then injected into a healthy animal, with the results as follows. what is the infectious agent? •amylase digests carbohydrates; tissue filtrate still infects healthy test animal. •lipase digests fats; tissue filtrate still infects healthy test animal. •formaldehyde and/or heat denatures dna and rna; tissue filtrate still infects healthy test animal. •trypsin digests protein; tissue filtrate does not infect healthy test animal.