Can a disease be detected before its symptoms appear? Explain the principle involved.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks whether a disease can be detected before symptoms appear and to explain the principle involved in such detection.

2. **Affirmative Answer**: Yes, a disease can be detected before its symptoms appear. This is crucial for early diagnosis and treatment.

3. **Challenges in Detection**: When a disease is in its early stages, the concentration of the pathogen (the disease-causing agent) is often very low. This low concentration makes it difficult to detect the disease using conventional methods.

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