Pneumonia can be caused by

Options

A. Bacteria ( Streptococcus penumoniae )
B. Protozoan
C. Fungi
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding Pneumonia**:
- Pneumonia is an infection that affects one or both lungs. It occurs when the body's natural defenses are compromised, allowing germs to invade and multiply in the lungs.

2. **Mechanism of Infection**:
- When germs invade the lungs, the body's immune response kicks in. White blood cells (WBCs) accumulate in the lungs to combat these invading organisms. This leads to the filling of the air sacs (alveoli) with WBCs, bacteria, and fungi.

3. **Causes of Pneumonia**:
- Pneumonia can be caused by various pathogens, which can be categorized into three main types:
- **Bacteria**: The most common cause of pneumonia. An example is *Streptococcus pneumoniae*.
- **Fungi**: This type of pneumonia is referred to as mycoplasma pneumonia.
- **Viruses**: Viral pneumonia is another category, caused by various viruses.

4. **Other Possible Causes**:
- In addition to bacteria, fungi, and viruses, pneumonia can also be caused by protozoans, although this is less common.

5. **Conclusion**:
- Therefore, pneumonia can be caused by bacteria, fungi, viruses, and protozoans. The comprehensive answer is that pneumonia can be caused by all of these factors.

### Final Answer:
Pneumonia can be caused by bacteria, fungi, viruses, and protozoans.

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