W32/Bugbear-B caused the biggest outbreak of the month, starting on 5th June. It can spread by email or across Windows shares, it attempts to kill many anti-virus and security programs, and it has backdoor functionality.
The list of W32/Sobig variants also grew steadily longer during June, with variants C, D and E discovered early on 1 June (Hong Kong time), 18 June, and 25 June respectively. Each variant so far is programmed to stop spreading on a certain date, W32/Sobig.E, currently listed as the most prevalent virus by MessageLabs, will stop spreading on 14 July 2003.
The persistentW32/Klez.H is still very common; MessageLabs currently list it as no. 2.