The correct answer is option B, Kala-azar.

Kala-azar, also known as visceral leishmaniasis, is a serious and potentially fatal disease caused by the protozoan parasite Leishmania donovani. It is transmitted to humans through the bite of infected female sand flies of the genus Phlebotomus in the Old World or Lutzomyia in the New World.

Here is a detailed explanation of why sand flies act as vectors for Kala-azar:

**Sand Flies:**

1. Sand flies belong to the family Phlebotomidae and are small, blood-sucking insects.
2. They are widely distributed in tropical and subtropical regions around the world.
3. Female sand flies require a blood meal to develop and lay their eggs.

**Leishmania Parasite:**

1. Leishmania donovani is the species of Leishmania responsible for causing Kala-azar.
2. These parasites live and multiply within the cells of the host's immune system, particularly in macrophages.
3. Infected humans and animals serve as reservoirs for the parasite.

**Transmission Cycle:**

1. The transmission cycle of Kala-azar involves two main hosts - the sand fly vector and the human or animal reservoir.

2. Infected sand flies become carriers of Leishmania parasites when they bite an infected human or animal reservoir.

3. The parasites multiply and develop within the sand fly's gut.

4. When an infected sand fly bites a healthy individual, it injects the parasites along with its saliva into the skin.

5. The parasites then invade the host's macrophages and multiply, leading to the development of the disease.

**Role of Sand Flies as Vectors:**

1. Sand flies act as biological vectors for Leishmania parasites, meaning they play a crucial role in the transmission of the disease.

2. As the sand fly feeds on the blood of an infected reservoir, it ingests the parasites along with the blood.

3. The parasites then undergo development and multiplication within the sand fly's gut, eventually reaching the infective stage.

4. When the sand fly feeds on a new host, it injects the infective stage parasites into the skin, leading to infection and disease development.

**Conclusion:**

In conclusion, sand flies, particularly those of the genus Phlebotomus in the Old World and Lutzomyia in the New World, act as vectors for Kala-azar or visceral leishmaniasis. They acquire the Leishmania parasites by feeding on infected reservoirs and transmit them to new hosts through their bites. Understanding the role of sand flies as vectors is essential for implementing effective control measures to prevent the spread of Kala-azar.

Kala Azar(leishmaniasis) caused due to the pathogen Leishmania Donovan which is spread by sand fly