In the search to discover the agents that caused mad cow disease, scrapie in sheep, and CJD and kuru in humans, the diseased brain tissues were filtered and still found to be infective, indicating they are not bacteria but either viruses or chemical molecules. If it was a virus, the infective agent would contain either RNA or DNA. Other possibilities were that the agent was a carbohydrate or a fat or a protein. Infective tissues 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 infective agent?

•Amylase digests carbohydrates; tissue still infects healthy test animal.
•Lipase digests fats; tissue still infects healthy test animal.
•Formaldehyde and/or heat denatures DNA and RNA; tissue still infects healthy test animal.
•Trypsin digests protein; tissue does not infect healthy test animal.

Based on the given information, we can deduce that the infective agent causing mad cow disease, scrapie in sheep, and CJD and kuru in humans is a protein.

Here's how we arrived at this conclusion:

The experiment involved treating the infective tissues with different agents and observing the results when injected into a healthy animal.

1. Amylase digestion: Amylase is an enzyme that digests carbohydrates. Since the tissue still infects the healthy test animal after being treated with amylase, we can conclude that carbohydrates are not the infective agent.

2. Lipase digestion: Lipase is an enzyme that digests fats. Similar to the previous case, the tissue still infects the healthy test animal after being treated with lipase, indicating that fats are not the infective agent.

3. Formaldehyde and/or heat treatment: Formaldehyde and heat are known to denature RNA and DNA, rendering them ineffective. However, even after treating the tissue with these agents, it still infects the healthy test animal. Therefore, we can eliminate RNA and DNA as the infective agent.

4. Trypsin digestion: Trypsin is an enzyme that digests proteins. When the tissue is treated with trypsin, it no longer infects the healthy test animal. Based on this result, we can conclude that the infective agent must be a protein.

Hence, the infective agent causing these diseases is most likely a protein.