How do I build a new, balanced, disease in D&D4e?

Constructing diseases and the underlying mechanics for diseases seem to be under-described in D&D4e.

  • How do I set a level for a new disease I want to create?
  • How do I choose difficulties for the tests for healing/disease progression?
  • How do I determine balanced consequences for the various disease stages?
  • What, if any, rewards should get attached to a disease?