Bagle (also known as Beagle) is a mass-mailing computer worm affecting all versions of Microsoft Windows. The first strain, Bagle.A, did not propagate widely. A second variant, Bagle.B, is considerably more virulent. The initial strain, Bagle.A, was first sighted on January 18, 2004. It was not widespread and stopped spreading after January 28, 2004. The second strain, Bagle.B, was first sighted on February 17, 2004. It was much more widespread and appeared in large numbers; Network Associates rated it a "medium" threat. It is designed to stop spreading after February 25, 2004.