Figure 1.

(A) The evolution and radiation of coronaviruses in humans and domestic mammals (via potential wild intermediate spillover host species). The radiation suggests there could be a vicious cycle of coronavirus emergence, whereby newly emerged viruses in new hosts increase the likelihood of producing more new recombinants. Red, blue and yellow arrows indicate the direction of spillover of coronavirus emergence for Alphacoronaviruses, Betacoronaviruses, and Deltacoronaviruses, respectively. Solid arrows represent direct (confirmed or suspected) coronavirus transmission between host species (although indirect transmission via an unidentified intermediate host is not excluded), and dashed arrows represent suspected indirect transmission via an unidentified intermediate host (although direct transmission is not excluded) (10, 17, 104, 127, 139, 141, 156). Dotted arrows with a question mark indicate uncertain spillover events. (B) A simplified phylogeny of the coronaviruses covered in this review, drawn from published findings (5, 129, 157).