Since mid-May the number of MERS cases reported by Saudi Arabia has fallen sharply, mostly due to a lack of any recent hospital-related outbreaks.
Sporadic community acquired cases continue to appear however, with two new ones reported today.
The case today in Madinah is attributed to indirect camel contact, and we'll have to wait for clarification on that - but as happens with so many other primary cases - the source of infection for the Riyadh case is unknown.
One of the big unanswered questions about the MERS coronavirus is what role – if any – do asymptomatically infected individuals play in the spread of the virus? While unproven, we've seen some evidence suggest they could be a factor (see Study: Possible Transmission From Asymptomatic MERS-CoV Case).
Last November's EID Journal: Risk Factors For Primary MERS-CoV Infection, Saudi Arabia found camel exposure a significant factor for infection, but that many community cases had no obvious exposure risk. They wrote:
Other potential explanations of MERS-CoV illness in primary case-patients who did not have direct contact with dromedaries include unrecognized community exposure to patients with mild or subclinical MERS-CoV infection or exposure to other sources of primary MERS-CoV infection not ascertained in our study.
But for a large proportion of community acquired cases, the source of infection remains a mystery.