If you consider a single body, the selection pressure is for the virus to multiply as quickly as possible. It's when you get multiple bodies that the selection

If you consider a single body, the selection pressure is for the virus to multiply as quickly as possible. It's when you get multiple bodies that the selection pressure is more the ability of the virus to be transmitted and infect a new host. It's by concentrating animals/people into smaller environments that we select for the greater transmission of the virus and possibly for it to be less deadly as that gives the virus more time to spread itself. Alternatively, think that a virus in a single organism is likely to be extremely well adapted for spreading within that organism already, but it may not be adaptable to other organisms and thus not be very contagious.