As explained by his website: "George Michael has severe community acquired pneumonia and is being treated as an inpatient. His condition has stabilized and he is responding to treatment."
Although seeming to get better, the release continues: "from the current point of view, the time until recovery cannot be estimated, but he will not be able to perform the rest of the tour. Besides medical treatment, complete rest and peace and quiet are mandatory."
As for what George fans are to do with their now useless tickets?
The site further details: "Discussions and preparations are currently under way in order to arrange the rescheduled dates and further announcements will be made at the earliest opportunity. In the meantime, tickets holders are asked to retain their tickets."
Keep the faith and get well soon, George!