A network based anti-virus system especially for wide area networks, like the Internet. In the system when a client computer downloads files or other web content from the network, their identifications are sent to an anti-virus host computer which compares them to a database of the identifications of known virus infected files or other known virus infected web content. Security measures are taken if a sent identification has a close enough match in the database. The system includes optionally an intermediate computer, like a LAN server, an ISP's server, network node computer or the source host computer of the downloaded files or other web content; which delivers the identifications to the anti-virus host computer on the behalf of the client computer. Security measures include the client computer or the intermediate computer preventing the download of the detected dangerous files or other web content to the client computer, or the client computer deleting the same from the client computer if they have already been downloaded; the client computer, the intermediate computer or the anti-virus host computer scanning the same for viruses when needed; and the client computer scanning itself for viruses when needed.