Name the causal agent of typhoid.

Explanation

### Step-by-Step Solution

1. **Understanding the Question**: The question asks for the causal agent of typhoid. This means we need to identify the specific microorganism responsible for causing the disease.

2. **Identifying the Pathogen**: The causal agent of typhoid is a bacterium. Specifically, it is known as **Salmonella typhi**.

3. **Classification of the Bacterium**: Salmonella typhi is classified as a **Gram-negative bacterium**. This classification is based on its cell wall structure, which does not retain the crystal violet stain used in the Gram staining procedure.

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