The emerging timeline for highly pathogenic viruses and the proposed Disease X. b. Schematic representation of SARS-CoV-2 S protein. Its S1 subunit contains NTD (14-305 aa), RBD (319-541 aa), and RBM (437-508 aa). Its S2 subunit contains FP (788-806 aa), HR1 (912-984 aa), HR2 (1163-1213 aa), TM (1214-1237 aa) and CP (1238-1273 aa). c. The formation of syncytium in Huh-7 cells 24 h after SARS-CoV-2 infection, with scale bar of 200 μm. 