Why diabetes mellitus is risk factor tuberculosis?

Immune function. Diabetes, especially uncontrolled diabetes, causes neutrophils to not behave as well as they should, so infections are both more likely and more frequent. Tuberculosis is one of these, and tends to be more severe in a poorly controlled diabetic.
Infection. Diabetes, particularly if your glucose is now kept near normal, increases the risk of all kinds of infections, including TB which is infectious. Your immune system doesn't function as well when your glucose is abnormal.

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