Explanation:

- Typhoid fever is an infectious disease.

- It is caused by the bacterium *Salmonella Typhi*.

- Symptoms include high fever, weakness, stomach pains, and loss of appetite.

- It spreads through contaminated food and water.

Here's a breakdown of the options:

1. Viruses: These are not involved in typhoid fever. They're responsible for illnesses like the flu.

2. Protozoa: These are single-celled organisms causing diseases like malaria, not typhoid.

3. Bacteria: This is the correct answer. Typhoid is caused by bacteria.

4. Fungi: These cause infections like athlete's foot, not typhoid.