Bit-torrent isn't the main
way to spread viruses. The main way to spread viruses is to use a more decentralized version of P2P like the Gnutella network (aka Kazaa, Shareaza etc..).
Bit-torrent is actually very useful and effective way to share files especially files that have an enormous size. (> 1 Gigabyte).
You can download any distribution of Linux you would like via BT (Bit-torrent), some companies issue updates to their software or games via BT. Blizzard distributes updates for World of Warcraft using BT, and for the game itself.