Concept: Kala-azar, also known as visceral leishmaniasis, is a serious parasitic disease. Understanding the type of microorganism that causes it is important. Step 1: Identifying the Causative Agent of Kala-azar
Kala-azar is caused by a type of parasitic protozoan belonging to the genus {Leishmania}. The most common species causing visceral leishmaniasis is {Leishmania donovani}.
Protozoa are single-celled eukaryotic microorganisms. Step 2: Transmission of Kala-azar
The {Leishmania} parasites are transmitted to humans through the bite of infected female phlebotomine sandflies. Step 3: Analyzing the options
(1) protozoan: Correct. Kala-azar is caused by {Leishmania} protozoa.
(2) fungus: Fungi cause diseases like ringworm, athlete's foot, or candidiasis. Not Kala-azar.
(3) bacteria: Bacteria cause diseases like tuberculosis, cholera, or typhoid. Not Kala-azar.
(4) virus: Viruses cause diseases like the common cold, influenza, measles, or AIDS. Not Kala-azar. Therefore, Kala-azar is caused by a protozoan.