Copyright infringement detection is a critical problem in large-scale online video sharing systems: the copyright-infringing videos must be correctly identified and removed from the system to protect the copyright of the content owners. This paper focuses on a challenging problem of detecting copyright infringement in live video streams. The problem is particularly difficult because i) streamers can be sophisticated and modify the title or tweak the presentation of the video to bypass the detection system; ii) legal videos and copyright-infringing ones may have very similar visual content and descriptions. We found current commercial copyright detection systems did not address this problem well: a large amount of copyrighted content bypasses the detection system while legal streams are taken down by mistake. In this paper, we develop the StreamGuard, an unsupervised Bayesian network based copyright infringement detection system that addresses the above challenges by leveraging live chat messages from the audience. We evaluate StreamGuard on real-world live video streams collected from YouTube. The results show that StreamGuard is effective and efficient in identifying the copyright-infringing videos.