The best replacement for mellitus is diabetes insipidus.

The best alternatives to mellitus are rated as following:
★★★★★  diabetes insipidus
★★★  insipidus
★★  diabetes
★  gestational diabetes
★  diabetes type 2
★  type 2 diabetes
  type 2 diabetes mellitus
  diabetes ketoacidosis
  type ii diabetes mellitus
  type 1
  type 2

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Speaking about mellitus, one probably shouldn't forget about tuberculosis.

Other search terms related to mellitus and their ratings are:
★★★★★ tuberculosis
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mellitus
diabetes insipidus
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