There's a New One. Besides the expected mild, moderate, & severe very nicely described by dr. Nash, there is now officially a 4th category --- mild cognitive impairment (mci). This is a stage before mild & the thinking is that here would lie your best bet at preventative strategies (if & when we find some of those).
Depends. There are many different systems used for different purposes: researchers or placement needs or hospice qualification.... Clinically, it can be helpful to use a simpler way of looking at it: mild illness is when a person has problems but can function with minimal supports, moderate disease leads to a need for major support and severe disease requires 24 hour care in a secure setting.