Assertion: Sandfly transmits kala-azar protozoa.
Reason: In kala-azar, the sandfly is an intermediate host.

Assertion statement:

1. Leishmania are flagellated protists that are transmitted by infected sand flies.
   
2. Kala-Azar or dum dum fever is also known as visceral Leishmaniasis caused by Leishmania donovani.
3. Hence, the assertion is true.
   

Reason statement:

1. All species of Leishmania use female sand flies as an intermediate host.
   
2. The parasite Leishmania is known to live and multiply inside a female sandfly.
3. The infected phlebotomine female sand fly finds a suitable host and inject the promastigote parasites into the skin during a blood meal.
4. The promastigotes are elongated, flagellated and infective parasites that grow in the midgut of the female sandfly.
5. In this way sandfly is intermediate host and human being is the primary host.

Final Answer: Both assertion and reason are true and the reason is the correct explanation for the assertion.