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A more-deadly virus ruins its chances only if it kills before it spreads. In this case, where it appears that COVID-19 is contagious for a week before substantial symptoms appear, and a large fraction of those who catch it are not debilitated, ultimate mortality has little effect.

Smallpox was overwhelmingly more deadly, and its spread was not limited, even without airborne transmission. Measles was also overwhelmingly more deadly, did rely on airborne transmission, and afflicted us century in and century out.

Research into viral function, absent military intent, has no reason to experiment with making the infection more deadly. The primary research goal is to discover what may affect virulence, and try to anticipate and understand what may occur naturally, and then discover ways to block those.