I ain't no virologist but it seems to me that Coronavirus' infectivity is inversely proportionate to its lethality.
Ranging from MERS with 35% mortality (R value typically <1), original flavour SARS with 10% mortality (R value 0.5 to 1), to SARS CoV-2 (Omicron) with <0.1% mortality (R value 4-5).
I suspect greater infectivity is associated with more infections in the nose/throat with less respiratory sequelae but larger amounts of virus ejected by cough/sneeze, and more mortality associated with variants that infect deeper in the respiratory tract but less prone to spray droplets everywhere.
This is of course likely to be overly simplistic, and it is entirely possible to be completely wrong!
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