https://sfist.com/2020/07/19/170-people-possibly-infected-with-covid-in-yosemite-valley-after-virus-found-in-sewage/ 170 People Possibly Infected With COVID in Yosemite Valley After Virus Found in Sewage The great outdoors is hailed as a recreational safe haven amid the pandemic. But after Mariposa County health officials shipped samples of untreated local wastewater to a Massachusetts lab for analysis, the yielded results show 170 people might have been infected in Yosemite Valley.
请问，这是怎么推断出的呢？ 有什么科学准则吗？
https://sfist.com/2020/07/19/170-people-possibly-infected-with-covid-in-yosemite-valley-after-virus-found-in-sewage/
170 People Possibly Infected With COVID in Yosemite Valley After Virus Found in Sewage The great outdoors is hailed as a recreational safe haven amid the pandemic. But after Mariposa County health officials shipped samples of untreated local wastewater to a Massachusetts lab for analysis, the yielded results show 170 people might have been infected in Yosemite Valley.