Recent studies had identified how the MERS-CoV recognizes and invades human cells: its spike protein binds to DPP4, a protein on the surface of human cells, and this leads to internalization of the virus which then takes over the human cell and turns it into a virus factory. Variations in DPP4 between animal species seem to determine susceptibility to MERS-CoV infection. For example, mice, hamsters, or ferrets, whose DPP4 is quite different from the human protein, seem resistant to the virus while rhesus macaques, whose DPP4 is very similar to the human one, are susceptible. However, while they can be infected with MERS-CoV, they develop only mild-to-moderate symptoms, unlike many human patients that carry very high loads of virus, get seriously ill, and sometimes die.