Common dedicated hosting
The article "Ensuring website safety" is provided by Sophos Plc and SophosLabs.
This hosting type is probably the most wide-spread and the most vulnerable to attacks It uses a dedicated server that hosts several sites. This hosting type is one of the most economical, which also makes it one of the most dangerous. Infecting just one server user is enough to infect all the others.
A good real example of the problems associated with shared hosting is described in the SophosLabs blog entry: http://www.sophos.com/security/blog/2007/06/172.html