To me, deadly should mean "how many people did it kill in a given time period." Rabies kills basically everyone who gets it, but I would never call rabies more deadly than covid, because barely anyone ever gets rabies. Rabies just can't be very deadly because it can't infect many people, while covid is very deadly because it infects lots of people. If two diseases have the same infection fatality rate, but one is more infectious, it would be silly to say that it isn't more deadly.

Obviously, it's why we have specific definitions like case fatality rate and infection fatality rate.

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