Just because a site is reputable doesn’t mean you’re completely safe on it – and YouTube is no exception. 300 hours of video are uploaded to YouTube every minute, a staggering statistic. Combine this with the over one million advertisers who use the platform, and you get the idea of how much information YouTube needs to filter in order to keep links off the site.
http://www.makeuseof.com/tag/youtube-major-source-malware/