The whole 'long term illness' thing, is and presumably always will be, rumour. The explanations that generally apply to the WSJ editions being in poor quality and far apart are pretty much easier to swallow.
In WSJ, the editors are often considered cruel, forcing series to continue on past their mark (e.g. dragon ball going past the frieza saga, against toriyamas original intentions), or offering little to no breaks a year. Now around the end of YYH it is said that Togashi's sheer popularity was at a peak, and he could hold some control over what he did, else he could stop drawing in readers, something crucial at the dark period WSJ was nearing, or in, i forget. As such, he gets a good payrate, more breaks, and through his own choice was allowed to work with no assisstants, and most importantly, broke out of the forced continuation thing.
Then came his next series, hunter x hunter. With this he was further in with his life with the sailor moon mangaka than before, and spent as much time as possible with her, and even more once he had a child some time later. These long absences are usually applicable to this, as hunter x hunter takes massive breaks and yet loses no ground with shueisha due to the popularity of the series.
And the quality of his art is rapidly produced, by himself, for WSJ, in whatever bouts he manages, and has the final versions worked on in his breaks to be released in tankobon format.
A good example of this would be when he returned last october. he did the poor quality chapters for 10 weeks, then had another ten weeks off, in which he produced the final quality versions for the tankobon, released when he returned, for, and i love this, another ten weeks. hopefully this pattern continues.
but basically there you have it. theres also some use of his time where apparently he gets addicted to rpgs, but i dont know much about that.
hopefulyl that gives you more of an idea of what COULD very well be the case, and seems the closest answer to the truth, considering Shueishas 'Togashi-ism'
keep in mind that the illness is by and large fan speculation, the more common reasoning people are looking at nowadays is this one, though nothing is really confirmed for sure
after all, we wouldn't want to feel too sorry for someone who's not sick or injured when we could be wondering when the mysterious absence of Koji Inada is due to illness/injury for sure and he could get our sympathy
That cover enough ground to give an idea?
I think that he needs to recharge his Nen with an stungun in order to use Kammuru again. And Yupi seemed to like he found in Killua a worthy opponent. 
