For coronaviruses and influenzas, they always mutate to become more infectious but less deadly. But when we have no immunity to it, it can be quite deadly at first. But sometimes we just gotta get through it.
@ 41. Tend to disagree. Many reports of anthropologists or missionaries exposing an unreached tribe to a cold virus and the indigenous people having severe disease and death. The host changed with acquired immunity so they had asymptomatic disease. While the virus still had potential to cause severe disease in humans without acquired immunity.