It is obvious that using a P2P one has to request the file in order to download it - it is also obvious that without  opening that file you are not going to get "infected" - so it must then be equally obvious that requesting the file from a P2P means that you are eventually going to open the file - otherwise why bother?.

Is not so simple, warez users are very much expert than normal users, you have to be very stupid to just download a file and open it. Warez sites and P2Ps are full of virus, porn dialers and any kind of crap, so they know very well the risk that have and the preventive measures to be taken.
There is also Darwin's natural selection at work, who is stupid enough must learn his lesson formating the disk so, or he never more download any warez or next time he is more careful downloading something.

And they sure ain't gonna tell you which files are infected. 
They do it, P2P if full of text files informing which file has a virus or is fake
Virus is not a big problem with P2P, worst is with mp3s that have wrong tittles or movies.
Imagine downloading 700M of a movie and then discover that is other movie, a porn movie or the movie itself  in Chinese language with Russian subtitles,  c'est la vie,  throw in in the trash can and try one more time.

1. Attempts to spread by sharing infected files on the Gnutella file sharing network, even if the Gnutella software isn't installed on the compromised computer

In theory a virus can install a P2P server in your computer, but what files will go to share???
All your exe files???, and who will request exes files?, who wants to download taskman.exe ???
Normaly you have no instalation programs in your computer, you have installed programs that by itself are useless. People download full programs, most of them in zip or rar format or some CD image, so nobody will download nothing from your infected computer with exception of your mp3 or avi files, but there's no way to put a virus in a mp3 file.
The only exception to the rule are Daz products that by itself are a full product, but the question is, do you have your original Daz product in your disk? or you have only the installed product that is not more an exe file and the original exe product is only in your CD backup?
And who will want to download aiko3.exe????, only Poser users!  what importance has Poser in the whole world?, almost none!, beside the 3ds communities almost nobody knows what is Poser or even 3d..
Do you think that Symantec knows  that Daz3d exist or what it is?

The whole story, or is a lie in an temptative to attack P2P networks and users, or the maker of the virus was an idiot just because the virus will spread nothing using P2Ps protocols.
This virus only spreads by conventional channels as email, messenger and all the Windows security faults.

Stupidity also evolves!