To date, as at 22 January, official information has not been sufficient to determine the source of infection and risk of human-to-human transmission for the newly emerged 2019-nCoV. Two competing hypotheses are therefore proposed and compared as potential explanations for the characteristics of the current outbreak of 2019-nCoV infections within and outside mainland China. Scenario 1 comprises a large zoonotic spillover event starting in early December 2019, perhaps over a number of days or weeks, and very limited human-to-human transmission subsequently. Scenario 2 comprises a small zoonotic spillover event in early December 2019 followed by efficient human-to-human transmission. Evidence consistent or inconsistent with each of these two scenarios is outlined in the Table.