Koch's postulates are not applicable for:

Options

A. Viruses
B. Fungi
C. Protozoans
D. Spirochaetes

Answer

**Step-by-Step Solution:**

1. **Understanding Koch's Postulates**: Koch's postulates are a set of criteria established by Robert Koch to determine whether a specific microorganism causes a specific disease. The postulates include:
- The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
- The microorganism must be isolated from a diseased organism and grown in pure culture.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

2. **Identifying the Types of Pathogens**: The question provides four options:
- A. Viruses
- B. Fungi
- C. Protozoans
- D. Spirochaetes

3. **Analyzing Each Option**:
- **A. Viruses**: Viruses are unique because they are not considered living organisms. They cannot replicate independently and require a host cell to reproduce. This characteristic makes Koch's postulates inapplicable to them.
- **B. Fungi**: Fungi are living organisms and can be studied using Koch's postulates.
- **C. Protozoans**: Protozoans are also living organisms and can be examined under Koch's postulates.
- **D. Spirochaetes**: Spirochaetes are a type of bacteria and are living organisms, thus they can be studied using Koch's postulates.

4. **Conclusion**: Since Koch's postulates are not applicable to viruses due to their non-living nature, the correct answer is option A: Viruses.

**Final Answer**: A. Viruses

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