Very low concentration of a bacteria or virus (at a time when the symptoms of the disease are not yet visible) can be detected by

Answer

**Step-by-Step Solution:**

1. **Understanding the Problem**: The question asks how very low concentrations of bacteria or viruses can be detected when symptoms of the disease are not yet visible. This indicates a need for a sensitive detection method.

2. **Identifying the Method**: The most effective method for detecting low concentrations of nucleic acids (DNA or RNA) from pathogens is the Polymerase Chain Reaction (PCR). This technique amplifies small amounts of genetic material, making it easier to detect.

3. **Explaining PCR**: PCR works by taking a sample from a patient and isolating the genetic material. If the sample contains the genetic material of the bacteria or virus, PCR can amplify this material to detectable levels.

4. **Process of PCR**:
- **Sample Collection**: A sample (e.g., swab) is taken from the patient.
- **DNA/RNA Extraction**: The genetic material is extracted from the sample.
- **Denaturation**: The DNA is heated to separate it into two strands.
- **Primer Addition**: Specific primers that match the target bacteria or virus are added. These primers will bind to the specific sequences of the genetic material.
- **Extension**: DNA polymerase is added, which synthesizes new strands of DNA by adding nucleotides to the primers, effectively amplifying the target DNA.
- **Repetition**: This process is repeated for several cycles, exponentially increasing the amount of target DNA.

5. **Detection**: After amplification, the presence of the amplified DNA can be detected using various methods, such as gel electrophoresis or fluorescence, confirming the presence of the bacteria or virus even in low concentrations.

6. **Conclusion**: Thus, PCR is a powerful technique that allows for the detection of very low concentrations of bacteria or viruses, even before symptoms of the disease appear.

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