Also, "circumstantial" doesn't mean weak. It means "pertaining to circumstance". For example, the fact that the virus's origins are in the same area as the lab would be circumstantial evidence that it was created in that lab.
Many circumstances can point to a conclusion. Even the phrase "the smoking gun" which has been bandied about in the discussion of this article, is an example of circumstantial evidence. Circumstantial evidence is opposed by direct evidence, which would be eyewitnesses, video/photos of the thing taking place, etc. And eyewitness is actually one of the weakest forms of evidence because memory is faulty. So circumstantial evidence is usually the evidence that convicts a criminal.
To illustrate that, I like to point to my two favorite circumstantial convictions, Hans Reiser and Scott Peterson.
They convicted Scott Peterson based on buying porn, a dye job, and selling cars.
The convicted Hans Reiser of first degree murder of his wife, Nina, despite not even having any direct evidence that she was in fact dead. They only found the body after Reiser himself led them to Nina's body in exchange for pleading guilty to second-degree murder.
A cleaned car, a missing seat, and some books on murder investigations were the evidence they used. Entirely circumstantial. And they deduced that Nina was murdered from those same circumstances.