Peer-to-peer (p2p) networking technology has gained popularity as an efficient mechanism for users to obtain free services without the need for centralized serv-ers. Protecting these networks from intruders and attack-ers is a real challenge. One of the constant threats on P2P networks is the propagation of active worms. Recent events show that active worms can spread automatically and flood the Internet in a very short period of time. Therefore, P2P systems can be a potential vehicle for active worms to achieve fast worm propagation in the Internet. Nowadays, BitTorrent is becoming more and more popular, mainly due its fair load distribution me-chanism. Unfortunately, BitTorrent is particularly vul-nerable to topology aware active worms. In this paper we analyze the impact of BitTorrent on active worm propaga-tion in the Internet. We identify the characteristics that accelerate and decelerate their propagation, and develop a mathematical model of their propagation. We also provide numerical analysis results. This will help the design of efficient detection and containment systems.