Same way that they smell witches out in sub-Saharan Africa. If you are a witch-smeller, and you say you smell witches, and that so-and-so is a witch, the crowd just tears them apart.
Same thing with Omicron in the sewer. A man in a white coat announces that he’s examined the sewage and found Omicron in it. Then they shut everything down, make everyone wear masks, and shut all the schools and the “non-essential” businesses down. He’s got a white coat and a diploma, so you know he knows more than you, and submit. You wouldn’t want to feel muscle ache and fatigue, would you.
The man in the white coat doesn’t do it. The politician in the suit does it after paying the man in the white coat to say what he wants him to say.