The agents which are known to cause CJD are

Options

A. Protein particles
B. A class of bacteria
C. A class of viruses
D. Fungi

Answer

### Step-by-Step Solution:

1. **Understanding CJD**: Creutzfeldt-Jakob disease (CJD) is a rare and fatal brain disorder that is characterized by rapid degeneration of the brain.

2. **Identifying the Cause**: The primary agents responsible for causing CJD are prions. Prions are not conventional pathogens like bacteria or viruses; instead, they are misfolded proteins.

3. **Nature of Prions**: Prions can induce normal proteins in the brain to misfold, leading to a cascade of protein misfolding that results in brain damage. This process is associated with the formation of amyloid plaques in the brain.

4. **Evaluating the Options**:
- **Protein Particles**: This option refers to prions, which are indeed the causative agents of CJD.
- **Class of Bacteria**: Bacteria are living organisms and do not cause CJD.
- **Class of Viruses**: Viruses are also not responsible for CJD.
- **Fungi**: Fungi do not cause CJD either.

5. **Conclusion**: The correct answer to the question about the agents known to cause CJD is "protein particles," specifically referring to prions.

### Final Answer:
The agents which are known to cause CJD are protein particles (prions).

---