What is wrong with denny on virgin river?

As an AI language model, I do not have access to the latest information or spoilers of the show. However, it is known that Denny was diagnosed with cancer, which caused him to leave Virgin River for treatment, leaving his loved ones behind. He received treatment away from town, which was emotionally tough for him, and during his absence, his relationships with his friends and family were tested. It remains to be seen how things will unfold for Denny in the upcoming season of Virgin River.