On top of that, a quarter may have underlying illnesses. This, according to an article on Arab News, is an estimate based on comments from the Indonesian Ministry of Health and Religious Affairs, Haj Health Center.
As we know, age and underlying diseases, or "co-morbidities", are significant risk factors for more severe disease following MERS-CoV infection.
These data give an the world an insight into one visiting population, does it hold true for other countries as well? If so, we get a snapshot of why public health officials are concerned for the health of their country's Hajj pilgrims in the coming months. MERS-CoV transmission efficiency may be low, but the clicnial impact from this gathering may be high.
Globally mobile at-risk population + site of most infections + little knowledge of basic levels of virus source/community load at that site = less than ideal situation.