Viral disease is

Options

A. polio
B. plague
C. cholera
D. typhoid

Explanation

### Step-by-Step Solution:

1. **Understanding Viral Diseases**:
- Viral diseases are infections that are caused by viruses. Viruses are microscopic infectious agents that can only replicate inside the living cells of an organism.

2. **Identifying Examples of Viral Diseases**:
- Common examples of viral diseases include measles, chickenpox, smallpox, and polio. These diseases are specifically caused by viral infections.

3. **Analyzing the Given Options**:
- The options provided are polio, plague, cholera, and typhoid.
- We need to determine which of these diseases is caused by a virus.

4. **Classifying Each Disease**:
- **Polio**: Caused by the poliovirus, hence it is a viral disease.
- **Plague**: Caused by the bacterium Yersinia pestis, hence it is not a viral disease.
- **Cholera**: Caused by the bacterium Vibrio cholerae, hence it is not a viral disease.
- **Typhoid**: Caused by the bacterium Salmonella typhi, hence it is not a viral disease.

5. **Conclusion**:
- Based on the analysis, the only viral disease from the options provided is **polio**.

### Final Answer:
- **Polio** is a viral disease.