According to the Santa Clara County Covid-19 Cases Dashboard, new infections are running about 25 to 30 a day. According to the testing dashboard, the test positivity rate is 0.5%. Checking online for the false positive rate from the most reliable of the tests, I get a range of estimates which includes 0.5%. So we don't actually know from the test results whether anyone at all is currently infected.
When I first noticed that, I checked the death rate from the same source. It was running at about 17 deaths from Covid a week, which suggested that there were still infections. This morning, however, when I looked at the same dashboard, the death rate (7 day average) was shown as about 3. It wasn't a drop over time but a sharp change in the number given for past weeks, the same weeks that had shown a death rate of about 17 as of two days ago.
My first guess was that they had for some reason replaced per week figures with per day figures and not bothered to tell anyone. But then someone pointed me at a recent news story reporting that the county had concluded it had been over reporting deaths and had just reduced its estimate for total Covid deaths by 22%. The reduction from 17 to 3 is a lot more than 22%, but the rate of actual Covid deaths is much lower than it was a few months ago so the ratio of mistakenly identified Covid deaths to actual deaths might be much higher — if, for instance, they tested everyone who died, using a test that had a significant false positive rate, and counted anyone who died with Covid as having died of Covid — as, according to the article, they had been doing.
An infection rate near zero with apparent deaths and infections almost all false positives fits the pattern of a rapid decrease in both deaths and infections down to a low level, followed by a roughly constant level of both thereafter. The rapid decrease implies a reproduction rate below 1, which should have meant a continued fall.
So perhaps it stopped falling because it got to zero.