There are different types of Infrastructure, in my view. Firstly there is the classic Hardware, often called "Tin and Wires". For me Hardware covers things like: servers, blades, networks, storage and so forth. Then there is Operating System Infrastructure, which for me covers Windows, Linux, Unix etc and includes Windows Clusters, Domains, LDAP and so on. Then finally there is Software Infrastructure which for me is software that supports custom development: Oracle Databases, Microsoft SharePoint, Microsoft SQL Server, Oracle WebLogic and so on. Virtualisation just blends in with all of this, whether VirtualBox or VMWare but not forgetting network virtualisation products like Xsigo.
It is worth noting that there are sections elsewhere that are about infrastructure but have been grouped elsewhere on the menu, specifically these are: