In other words, not all of those people that had a virus are currently infectious. Only the people that are currently actively infectious are the ones that need to be traced.
I'm hearing such a wide variety of claims about this virus that I have by default become skeptical of anything that makes a wide ranging claim. The descriptions range from "highly infections, potentially causes strokes in otherwise young healthy people" to "highly infectious, young people often display no symptoms at all".
Are there multiple strains of this virus out there in the wild? I don't know of another way to explain what I'm hearing from people that work in virology and in hospitals on the front line.
There's probably a lot of other factors that feed into it - median age, pollution, weather/temperature, potentially genetic/ethnic factors... Even things like the prevalence and use of public transit. It's a lot harder to get/spread the virus in LA where most people drive private cars to everything than in NYC where a lot of people spend an hour or more every day in crowded subway trains/stations.
Working out exactly what contributed to the real-world outcomes we're seeing is something we'll probably only be able to guess at, but nothing I've seen so far feels inexplicable.
Both those things can be true at the same time.
I saw one German doctor (from Charité Berlin I think?) who did over 100 autopsies on TV saying in all cases he had on his table there were preexisting conditions (and the older the more likely you have one or more of those). But he also said that the young and "healthy" people he looked at also had preexisting conditions that usually were never diagnosed before.
This reminded me of a buddy in school, who went for the routine mandatory checkup before the mandatory military service (since been abandoned), a jock-type who seriously considered joining up voluntarily to become a jet fighter pilot, and ended up learning he only had one kidney since birth. Something he never knew or suspected and never had any issue with because he was otherwise young and healthy, but which could come back to bite him when he gets old or when he contracts some serious disease (like this one maybe).
In some sense that is pretty high compared to the baseline, but being equal to 10 years of driving isn't apocalyptic.
One of probably a dozen panel discussions on COVID19 he's hosted so far.
UCSF Experts on the Epidemiology, Science, & Clinical Manifestations of COVID-19, and UCSF Response