Some things are harder to cure than others. That's because they have more causes.   Multiple causes.
 

• Viruses are probably the easiest, because if you remove the virus from the right bodies till the physical symptoms are gone -- and with flu that usually takes only a few minutes -- it's gone and  doesn't come back.

With herpes, if you catch it and keep it out of the right bodies, you never get another outbreak, and at some point it never comes back into the higher bodies either.

Viruses have few causes.
  

• Cancer has more causes, and is therefore more difficult to root out.
    

• Diseases that fall into the category of immune dysfunction,  such as depression and addiction, and that are caused by other complex problematic codes such as conditions, are the hardest. Here's a list of typical immune dysfunctions.
   

• Here's a page on curing conditions, which cause immune dysfunctions.
   

- Buz