This is one nice and intelligent question.
Why Linux os does not need an antivirus program ?
We do not need antivirus for the following possible/ obvious reasons:
--Applications handling personal/business/critical data are being run in the $USER authority, unlike the other OS from Redmond. This could probably mean that viruses, if they may come into the /tmp or /var/tmp or wherever it can land still it is wanting favorable environment to infect others. It merely sleeps there until the moment Mr. Clamav will discover its hiding and purge it out. I have experienced this one many time. I run clamav only by weekly, before my cron backup runs. You can download Clamav antivirus from here
if you want, although it is not necessary still it is good for "mental health" to those who came from M$windows. In addition to this, my experience tells me that these trojans sleeping idle in the /tmp or wherever, they are gathered mainly from web clients and that notorious "wine" application; yet, having came into my system they are unable to move.
--Unix Philosophy is so good that, given a bonafide Gnu/Linux/*nix distro it should take a remote virus coder a hundred years to penetrate your $USER environment --he needs another malicious grandson to complete that work against you. noexec. nosuid. virus-no-way.
--Free Software Costs not a day's wage. Advanced Gnu/linux users hold no fear at breaking the system and install another only to break the latter and change to another distro again, so on and on; this means --here in the free and open software world we pick and trash OS's the luxury of a prodigal son, "complete FREEDOM without fear" the very opposite of "blue-screen-nightmare" capitalized in the microsoft horror world that devoured Billions of Dollars from the pockets of dull/lazy/simpleton users. The point is: get me a virus coder, if his grandson be as villain as he, he can break my Slackware 14, by the time I shall be running Version 33 (maybe, ...G-d bless PV)
--my data will no longer be there. In this kind of computing world virus coders are wasting their time.
There are more reasons than that, and even better explained by others who have the gift of eloquence.
almost all the leading antivirus vendors are already released AV program for linux..
Fear and ignorance: is an ancient capital for pecuniary enrichment. Companies simply have to follow their clients wherever they go just to make money. Most antivirus buyers from the Gnu/linux/*nix world are NOT the natives but the migrants from microsoft horror-world who carry with them the same "fear and mental dependence" to companies. Not only antivirus, sooner or later (as our freedom world increase in population) you'd be seeing "Backup Software for GNU/Linux/Unix @ $115 copyrighted to ONLY ONE system" --same foolishness against the fools-- when a simple terminal command "~$ tar czf myhome.tar.gz /home/$USER" can complete that work without losing a penny. Examine these types of backup software they're nothing but scripts of commands if, cd, cat, grep, rsync, tar, bzip2, etc. things that are just waiting under your fingers. The margin aimed by these antivirus companies are the post-M$ migrants coming to our freedom land. Don't wonder, that's the Law of Supply & Demand. So long as the man is lazy he is required to pay; and many will offer that solution --solutions to supplement the laziness.
So study Gnu/linux. Study how to escape from the trappings of these horror-mongers from the old world. They (the lazy) gain say "Owww Gnu/linux poses a high learning curve!" True a decade ago. But not today. Righteous men are making it easier and enjoying to learn Gnu/linux today, to mention some: Debian people, Canonical, RedHat, the Mint team, and not the least, the people from www.linuxquestions.org"
what are the basic protection mechanism having linux machines against viruses and malwares???
Avoid sinning and increase the good works.
Here they are:
login: USER <auto>
login: "root" <$PASSWD>
Pseudo good works:
login: "$USER" <$PASSWD>
@ /usr/bin/firefox <type URL> www.linuxquestions.org
Hope that helps.