Is it necessary to have a antivirus installed for linux?

No and yes.

No:
due to the low low low number of virus that target the linux platform, installing an anti-virus would be pretty useless.

Yes:
If you are running a mail server used by windows users or you dual boot you might to have anti-virus installed.

It's dependent on the purpose of your box, and what you want.

Thank you very much.

It doesn't hurt. Like anything, it depends on your needs. If you don't mind risking your system, don't bother with antivirus software. Then again, if you're like most people (cautious, but not paranoid) the extra effort is worth it. Clamav is a decent item, in this regard. (That is, when it doesn't trip on the GPL.)

For instance, out of the 14 servers I manage, I have two that are wide-open to the internet. One that is a multi-chrooted box with varying levels of security--it's funny to see how often a script-kiddie will try to "compromises" a system, completely oblivious to the possibility that someone might actually be observing them. The other is an "oops" box that's independent of the main network that I personally use to guage vulnerabilities.

While I'll never post a "challenge" for an attack (as some twit posted on LQ earlier), neither will I completely trust an important system to hold it's own against the creative minds out there. So, for the systems that pay my grocery bill, they all have antivirus and several levels of security insurance on them.

All in all, I'd say it's not entirely necessary to have an AV for Linux (yet), but against an active attacker, it's not a bad idea to protect yourself as best as you can. You know, the whole common sense thing, like the reason you don't get out of your car and leave the keys in the ignition all night.

Basically, it's not that much effort to use an open source AV, so why not?

* If you don't mind risking your system, don't bother with antivirus software. Then again, if you're like most people (cautious, but not paranoid) the extra effort is worth it.
* All in all, I'd say it's not entirely necessary to have an AV for Linux (yet), but against an active attacker, it's not a bad idea to protect yourself as best as you can.

If you're cautious and value your systems you should properly harden it.
Hardening starts by taking away chances (by removing vulnerable and unnecessary parts of the system), controlling access to the remaining parts of the system (restricting system capabilities, users, processes and service access) and making the system more, better auditable in general.
Antivirus software will not protect a system against crackers nor will it stop a cracker.

Actually I agree that even though now an antivirus is not need, we might need one in the future. Especially if linux goes mainstream and draws more users. But as I have mentioned, it's pretty useless for now.