Malnourished individuals have a weakened immune system, which impairs their ability to control Leishmania donovani infection, the causative agent of kala-azar (visceral leishmaniasis). This immunosuppressed state increases susceptibility to the parasite and progression to disease. Malnutrition particularly affects cell-mediated immunity, critical for controlling intracellular pathogens like Leishmania. Therefore, malnutrition is a major risk factor for the development and severity of kala-azar.

Reference: Harrison's Principles of Internal Medicine, Kasper et al., 20th Edition.