The tip of the VirusBerg...

The VirusBerg.
Click to enlarge. If there are more cases with mild/less
 symptomatic disease then the denominator grows
and the proportion of deaths to survivors, the PFC,
shrinks. At the start of an outbreak or emergence of
a new pathogen, it is usually the most severely ill or
those ill with a specifically identifiable disease
that attract the attention of specimen collectors &
testers. Later in the event as resources are freed up
and panic reduces, testing of other population
groups may find more cases of less severe disease.
 This may not eventuate though and the PFC may
 remain high confirming a serious threat to human
health. Feel free to use graphic. Cite this blog
and Ian M. Mackay.

I've talked about the Denominator problem over on my "main" site, Virology Down Under's H7N9 page, but I thought it worth revisiting for 2 reasons: (1) our understanding of Middle East respiratory syndrome (MERS-CoV)  fatalities may still affected by a possible bias in our understanding of MERS-CoV transmission because of a continued focus on testing just the "tip" of the VirusBerg and (2), I made a new graphic that I like more than the old one!

We see a high PFC for MERS for 2 reasons that spring to my mind:

1. There are no, or very few, mild cases of MERS. Cases are usually severe and most are being caught. We know there are some mild cases and we know that contacts of confirmed cases are infrequently MERS-CoV positive. I'm assuming contacts are tested with the same rigour as those in hospital due severe MERS (e.g. lower airway sampling with follow-up testing). 
2. Although we don't see much transmission from cases to contacts we do see some and this may be enough to maintain a small number of community transmission events. The second reason then is that we are only testing cases with severe disease, many of whom die as a result of complications due to the initial infection. Prospective screening of a sample of the well general community and those with "common colds" might identify more cases that would reduce the PFC.