Patients with diabetes or chronic renal failure at high risk for MERS...

A recent New England Journal of Medicine editorial piece, accompanying that huge MERS-CoV case study previously discussed here, notes the immediate concern of October's Hajj pilgrimage to the hot zone, Saudi Arabia.  

Perlman and McCray also note that MERS-CoV is a system-wide infection and that while the average number o new cases from each index case (R₀) is reportedly...and so far, practically (tertiary infections seem to be rare)...below 1, there are anomalous super-spreader events that draw attention to healthcare settings as the places to watch in the future. Similarly, those with diabetes or chronic renal failure are the populations to watch. 

The virus itself needs careful observation to see if/when it shows the telltale changes to its cell-binding proteins that indicate it may have shifted gear, becoming a more aggressive spreader among humans.

I have not been able to find much by way of virus epidemiology studies among those with severe illness and underlying conditions like those above, so its still very hard to know whether MERS-CoV is unique and scary in this role, or whether other, more endemic CoVs could wreak the same degree of havoc in this populations.