Hi Sandytrat.
You're right. Since months, usenet and torrents are flooded with millions of infected files. That's correct, you can search anything you want, you will find dozens of items that will match your query.
In very few words :
1. In the past, viruses were designed to destroy a target. Things have changed today. The "criminal" business has migrated on internet. And, their goal is not to disturb your activities, but steal something they want. And the more computer are infected, the more they can collect informations and the more they get money.
The "criminal" organizations grab, sort and re-sell stolen informations to a "customer". They use today the same economical model used in the legal economic world : objectives, project plan, marketing plan,development plan, teams ( code review team, testing teams, ... ) Don't imagine anymore a stand-alone developer geek in the dark bedroom.
And, thrust me, when the dev teams have finished coding, they all post themselves the badfiles on usenet and create tons of different poster name.
2. They use dedicated tools ( software suite ) developed in order to automate the compilation of malware ( trojan, spyware, all bad things you wantware ). It means that a new version of heuristic is each time generated ( simply said ). How can your antivirus could detect this bad file if the signature of the bad file is not yet known by your AV provider ? The criminals generate so many files in a day
It permit also, for example, to inflate the size of the file. We used to take care of few kilobyte files, but what do you think about a 25 megabyte file ? Infected file or not ? Difficult to answer, if your query was a small application. The badware is well designed : it doesn't stops anything, don't use huge network resources when transmitting datas, and, well done guys, it sends datas in encrypted format to http servers ( good luck to analyze what your host machine is sending ! )
3. Usenet and torrents are not regulated. It's the right place to be for a badfile provider
They flood tons of data collectors every day, every where. Again, The more they hit somebody ( oops, I ran the exe file ), the more they collect infos than cash.
But, official software provider also flood bad versions of their release in order to discourage leechers. Finally, after several computer re-installation, they will probably buy the stuff.
The best method is to estimate the credible size of your file ( ex : tv video file, 50 min, 300 Mb ) and think around : is it credible if, for example, I need an MP3 an after unrar, I get an exe file ?
Working with the dark side of internet leads to accept to change some habits :
Full host system backup
Virtual machine to test exe files, snapshot before, discard if issues.
Sandbox environment on several AV software
Cheers