I am reading these negative neural impacts ranging from bad to worse, and I cannot help but think, how come there is not something in nature, a virus or bacteria, on the whole planet that can do something good? I do not know help boosting confidence, making people happier, increasing cognitive abilities, maybe improving your sense of smell or direction, gain muscle... at least something, instead it is always a very long list of destruction. Sorry guys, I am trying to offset, reading this paper made me depressive.

EDIT: After reading all the answers, I remember a quote, probably answer to my question. It is not possible because we are very complex organism, and regardless how some virus or tiny bug can find flaws in our complexity, in the same way as randomly injected piece of code cannot improve application, the same way, is an equal chance that a virus will introduce positive change in our genome which will improve parameters we desire.

Quote was Hoyle's Fallacy used for something completely different: "the chance that a tornado sweeping through a junkyard might assemble a Boeing 747"

So it seems we need to wait for few million years or to really on marvel of genetically engineering.

p.s. Thank you guys!