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What is Which of the following organ is most severely affected in Kala azar? and explain in detail?
A disease caused by infection with leishmania parasites.
Visceral leishmaniasis / Black fever is spread by sandfly bites. 
This type of leishmaniasis affects the internal organs, usually the spleen, liver and bone marrow.
Some people have no symptoms.
For others, symptoms may include fever, weight loss and swelling of the spleen or liver.
If left untreated, severe cases are typically fatal, by affecting spleen.