The original source of infection and mode of transmission to humans is unclear. At least 2 cases were reported to have visited farms and may have had contact with animals [20, 32]. Thus a zoonotic infection is a possibility. Furthermore, the fact that the NCoV is most closely related to bat coronaviruses [3–5] indicates that it might have originated from bats. Studies showing that SARS-CoV was most likely to have derived from bats [33] also supports a zoonotic origin for this new coronavirus. However, it is unknown whether the NCoV was transmitted to humans by a direct interspecies jump or it involved another intermediary animal. In the case of SARS-CoV, civet cats were identified as a likely intermediate host [8, 9, 33]. Like SARS-CoV, the exact natural reservoir species of the NCoV also remains to be identified.

Most of the laboratory confirmed cases of NCoV do not appear to have had any contact with animals. Could there be human-to-human transmission? The two clusters of cases, one from a family in Saudi Arabia, the other from a healthcare team at a hospital in Jordan and the most recent UK cluster of cases certainly suggests that human-to-human transmission is very likely. In the family cluster, 3 members contracted the infection. The index case is thought to be a 70-year old grandfather admitted to a hospital in Riyadh with severe respiratory illness. His two sons took in turns to stay by him during his illness (Dr Ziad Memish, personal communication). In spite of all supportive care, the patient died of renal failure. Both of his sons subsequently also contracted the infection and one of them died of multi-organ failure some days later. In the Jordanian cluster, 11 cases (7 nurses and 1 doctor) presented with an acute respiratory illness of unknown aetiology in April 2012 [25]. Two of the cases subsequently died. Retrospective testing confirmed both cases to be positive for the NCoV [34]. Investigations of the non-fatal unconfirmed probable cases in this cluster revealed that the illness in these cases was generally mild [24]. Indeed, in one case, the symptoms were mild enough to be managed at home. The cases in both, the Saudi and Jordanian cluster had no reported contact with animals. Similarly, the most recent laboratory confirmed cases from UK (cases 11 and 12), also had no contact with animals or any history of travel to the Middle East. Both appear to have contracted the infection from a family member who had recently returned from Saudi Arabia [30]. Together, these anecdotal observations clearly indicate that person-to-person infection, probably via respiratory mode of transmission, is highly plausible.

Based on the data we have so far, NCoV does not appear to be highly infectious in humans. None of the first 3 confirmed cases (2 Saudis, 1 Qatari) have been associated with transmission of symptomatic disease to close contacts [20, 32]. Detailed follow-up of 64 close contacts of the Qatari patient, which included healthcare professionals, family members and friends, only 13 individuals developed respiratory symptoms, but none of these cases were positive for NCoV [32]. This would suggest that human-to-human transmission is limited.