Causation != correlation. Not all bad moods are caused by bacteria and you’re not factoring in the fact you were sick and got well as a potential mood boosting factor

I'm inclined to believe that GP is rational enough to be able to notice if there were symptoms other than low mood that were so severe and troublesome that GP reacted with chronically-low mood.

And I'm inclined to believe that GP is competent enough at writing comments that if there were such non-mood symptoms, GP would've mentioned them.

(I don't know anything personally about GP: I'm going on the average rationality level of the writers of HN comments that report on the writer's own health.)