Why might less-severe respiratory virus strains spread more successfully than severe strains?

In the context of Darwinian medicine, the level of virulence—the ability of a pathogen to damage host tissues—is often shaped by the requirements for successful transmission.1

For respiratory viruses, less-severe strains often spread more successfully because they allow the host to remain mobile and active. If a virus is so severe that it keeps the victim confined to bed, the pathogen's opportunity to encounter and infect new hosts is significantly restricted. Conversely, strains that cause milder symptoms enable the host to continue interacting with others, thereby facilitating the transmission of the virus to new individuals.1

This dynamic illustrates how natural selection acts on pathogen traits to maximize the rate of spread, rather than necessarily favoring the survival of the host or the mildness of the disease. This contrasts with other pathogens, such as malaria, where high virulence may actually enhance transmission by rendering the host too ill to defend against the mosquitoes that spread the parasite.1