Mortality Rate is Number Dead Divided by Number Infected so Why No Estimates of Denominator Rather Than Just Tested?

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Reportedly 4,700 people in the U. S. have died of the wuhan virus, in this nation of about 350,000,000 people, and the mortality rate of the wuhan is the number dead (4,700) divided by the number of those 350,000,000 who have been infected, not by the number tested positive for the wuhan (about 120,000) rather by the total number infected whether detected or not (most people show slight or no symptoms). The experts say that the wuhan is very contagious, having spread (more slowly now) over a three month period, so how many Americans have been infected, just a few million but much more probably tens of millions (forty million which is about 1/8th of the Country?), and thus what do the experts estimate is the mortality rate, maybe about 1 in 10,000 (0.01%), much like the common flu?