Kala-azar disease is caused by ______.

protozoa

The correct answer is Option (2) -protozoa

Kala-azar, also known as visceral leishmaniasis, is a disease caused by protozoan parasites of the Leishmania genus. These parasites are transmitted to humans through the bites of infected female sandflies of the genus Phlebotomus in the Old World or Lutzomyia in the New World.

Once the parasites enter the human body, they invade various organs, including the liver, spleen, and bone marrow, where they replicate within macrophages, a type of immune cell. This invasion can lead to a range of symptoms, including fever, weight loss, enlargement of the liver and spleen (hepatosplenomegaly), and anemia.