Which of the following parasites is nonliving outside the host
but becomes living on entering it?

Options

A. (a) Virus
B. (b) Bacteria
C. (c) PPLO
D. (d) None of these

Answer

To solve the question "Which of the following parasites is nonliving outside the host but becomes living on entering it?", we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: We need to identify a parasite that is nonliving when outside a host organism but becomes living once it enters the host.

2. **Analyze the Options**: The question provides multiple options (though they are not listed here). We will evaluate each option based on their characteristics.

3. **Evaluate Bacteria**:
- Bacteria are living organisms that have their own mechanisms for reproduction and metabolism.
- They do not require a host to survive and can live independently outside of a host.
- Therefore, bacteria do not fit the criteria of being nonliving outside the host.

4. **Evaluate P-P-L-O (Pleuropneumonia-like organisms)**:
- P-P-L-O, which are similar to mycoplasma, also have their own genome and metabolic processes.
- They can survive outside of a host and do not depend entirely on a host for their existence.
- Hence, they also do not meet the criteria.

5. **Evaluate Viruses**:
- Viruses are unique in that they are considered nonliving outside of a host. They do not have metabolic processes or the ability to reproduce on their own.
- They require a host cell to become active, reproduce, and carry out their life cycle.
- Once a virus enters a host, it hijacks the host's cellular machinery to replicate and become "living" in a sense.

6. **Conclusion**: Based on the evaluations, the only option that is nonliving outside the host but becomes living upon entering it is the virus.

### Final Answer:
The correct answer is **A. Virus**.

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