Which organism causes Kala-azar ?

Explanation

**Step-by-Step Text Solution:**

1. **Identify the Organism**: The organism that causes Kala-azar is the protozoan parasite known as *Leishmania donovani*.

2. **Transmission**: This parasite is transmitted to humans through the bite of infected female sandflies, primarily of the genus *Phlebotomus*.

3. **Symptoms of Kala-azar**: The disease is characterized by fever, weight loss, swelling of the spleen and liver, and anemia.

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