Is there any real-time anti virus for Ubuntu? Which antivirus is good and in real-time (I mean that, if I try to open an infected file or go to a bad website, it stops it).
I read a post in ubuntuguide and here is a part of it:
Anti-virus

If you are running a file server, interface frequently with Windows drives or share files with Windows users, or use virtualization, you will want a virus checker for your Windows files.

Despite extensive misinformation, Linux is not immune from malware (witness the explosion of malware being created for the Linux-based Google Android systems). The malware is not usually spread within the OS itself (as long as the OS is a well-respected distribution obtained through official channels), but in trojan programs downloaded and installed by users outside of the normal software distribution channels (i.e. repositories) of the OS. There is always a danger to using programs downloaded from the Internet from sources other than respected repositories -- it is the primary reason that Debian and (K)Ubuntu retain tight control over their software repositories.

Any file can have malware embedded in it (which is trivial to achieve by concatenation, for example: cat originalfile.avi malware.exe > originalfileplusmalware.avi). The question is whether a user will try to open a file with a program (such as a media player) that has been compromised in a way that allows it to execute the code found in the infected media (e.g. .avi) file. This can occur not only for Windows users but for any OS (including Mac OSX and Linux) with a compromised program (e.g. media player). An example is the extensive problems the Mac OS community is currently having with the Flash player.

Routine scanning of any file downloaded from the Internet, any file imported from another user's computer (even a trusted source, since their attention to virus prevention may not be as compulsive as yours), or any attachment received in an email (even from a trusted sender) should be done with an anti-virus program. So I think that there is a need for anti-virus in Ubuntu.