Bird flu is caused by

Options

A. protozoan
B. bacteria
C. virus
D. helminth

Answer

**Step-by-Step Solution:**

1. **Understanding Bird Flu**: Bird flu, also known as avian influenza, is an infectious disease caused by influenza viruses that primarily infect birds.

2. **Identifying the Cause**: The causative agent of bird flu is an influenza virus. This means that the disease is viral in nature.

3. **Types of Bird Flu**: There are more than a dozen types of bird flu viruses that have been identified. Among these, two notable strains are H5N1 and H7N9.

4. **Human Infection**: Although bird flu primarily affects birds, it can occasionally infect humans, but such cases are rare.

5. **Conclusion**: Based on the information gathered, the correct answer to the question "Bird flu is caused by" is "virus".

**Final Answer**: Bird flu is caused by a virus.

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