What's hard to understand about camels carrying MERS-CoV and rarely infecting humans...?

What is the hard part to understanding that camels harbour the same Middle East respiratory syndrome coronavirus (MERS-CoV) that infects humans, but only does so rarely? Not a different strain or species - variants of the same virus.

It may be that this is not a simple 1 + 1 = 2 kind of equation and sometimes that can be a tough camel to ride. 

There is a need to understand a few things where camels are concerned. For example...

1. Camels definitely get infected by MERS-CoV - they can get mildly ill or not. Infection effectively results in a camel "common cold" illness
2. When a camel in a herd is infected, that doesn't mean that every camel in that herd is infected
3. When a camel is infected it may not be very ill, or show no sign of illness at all
4. MERS is a respiratory disease - while there is no evidence for exactly how humans acquire MERS-CoV, it has been considered, by medical experts in the Kingdom of Saudi Arabia, to be most likely acquired via droplets or other modes of transport of virus contaminated material and the upper or lower respiratory tract epithelium (lining). Ingestion is not considered to be a likely route of infection to date. This may be why those who drink fresh camel milk do not all get infected by MERS-CoV. But frothy bowls of milk have lots of popping bubbles that could create droplets that can be inhaled. And MERS-CoV can survive in milk and in the cold and on surfaces (which can be contacted and then self-inoculated via eye rubbing, nose picking etc). But the distinction between ingestion and inhalation can be confusing.
5. Most human MERS cases have not reported camel contact. Most cases have acquired their infection in association with a small or uncontrolled hospital outbreak of disease
6. In the 185 cases of MERS-CoV infection acquired in South Korea - none were infected by or had any, camel contact. Those cases were due to human-to-human infections. Camel contact is a sporadic cause of infection despite most camels in the Arabian Peninsula showings signs of past infection
7. Because camel contact is rare does not mean it never
   occurs - just that it is rare. A single camel-to-human
   
   infection may trigger dozens or more human cases if the hospital which that first case attends does not have effective infection prevention and control procedures. We have seen this again and again and again since 2012
8. Those in close contact with camels, who are otherwise healthy, may have been infected but not developed more than a cold or flu-like illness (who does anything about those - or remembers when they had them?). 
9. We do not yet know whether those who are in frequent close contact with camels and who have underlying disease, may also have some cross-protective immunity due to infection by a closely or perhaps even distantly related camel virus that does not cause lethal infections, as MERS-CoV does, in those with a comorbidity 
10. MERS-CoV may move around Africa and the Arabian Peninsula via infected camel imports and exports but no surveys of camels in Africa for MERS-CoV, or other coronavirus ancestors, have been reported to date
    
11. No MERS-CoV PCR-based diagnostic surveys of respiratory disease cases - mild, moderate or severe - have been conducted in countries harbouring camels known to have been infected by MERS-CoV (or an antigenically similar virus) in the past. Ethiopia the Sudan and Somalia are such countries
12. Camels do not need to be culled to prevent infection - they just need to be approached with more awareness and appropriate care to reduce the risk of infection and disease. Plenty of animals that we co-exist with carry viruses that can infect us and seriously harm us - we don't seek out and kill them all to stop getting infected, we need to address the activities by which we humans get infected.