There are Linux antivirus products. There are just far fewer Linux antivirus programs when compared with antivirus programs for Windows. There are a few reasons for this:
Virus writers want attention. Since most people that interact directly with the OS interact with Windows, why not make a virus to infect Windows since you will annoy the most people?
Stereotypically, a larger proportion of Windows users are vulnerable to making mistakes that allow viruses to infect their system when compared to Linux users.
Windows is more vulnerable to attack. This is a very debatable topic, but it is widely believed that there are more exploitable holes in Windows than there are for viruses.
It is also argued that potential holes are found and patched in Linux versions faster than in Windows versions because of the fact that more people look at, analyze, and test the actual code that builds Linux distros and much of the software that runs on them. This is due the the fact that most of Linux software is open and able to be viewed by anyone.
It should be noted that while Linux doesn't suffer from most of the software considered to be viruses, there are still large batches of software that "infect" Linux machines. The worst kind of software to get on a Linux machine is a rootkit. Rootkits allow someone to gain top-level system access and control while not alerting the actual users of the system that anything has changed. These rootkits can modify and spy on data or run programs in the background without any of the processes or data accesses appearing to the legitimate users. The fact that another user logged into or is still logged into the machine could also be completely hidden. Other kinds of malicious software and data that can find its way on Linux machines are loggers, poisoned DNS entries, backdoor botnets, harvesters, spam gateways, etc.
As you can see, Linux machines are not free from security concerns. The main difference between security on a Windows machine and security on a Linux machine is how you keep a high level of security. In Windows, you want to make sure that you have a strong firewall, observant antivirus, thorough antispyware/adware, and careful eye on what to watch for on your system. With a Linux machine, you want to configure a tight firewall, run nightly updates (up2date, etc) to ensure that critical patches are installed and software is updated, ensure that all unnecessary daemons and programs are disabled and locked away, and that all code added to the system is secure and monitored for abnormal behavior/use. The differences mainly stem from the fact that most Windows machines are used as desktop productivity or gaming machines while most Linux machines are used as different types of servers (web, email, DNS, proxy, FTP, IRC, etc).
Actually as far as I know, most of the people who use Linux or depend on it are a little more introduced to computing and had already been a victim of a virus attack more than once on a Windows Machine. So they
know the loopholes and are less susceptible to virus attacks.
Another reason why virus writers write for Windows, is they aren't open source, and in the hacker community, they can tell others that they did this on a non-open source operating system. ALso, since Linux is open source, the holes are patched within a few hours - days.
there are alot more people trying to find exploits in Windows Os's than there are for any other OS type. FireFox is a good example, people thought it was way more secure than Internet Explorer untill it became super popular, now exploit finders are finding the same kind of issues with FF
Another consideration is the amount of 'STUFF' bundled with windows unlike *nix which is very modulated, the more there is the more there is to go wrong.
Windows is full of viruses because all hackers hate Bill Gates and love to attack the BIG GUY. Microsoft did NOT write their code based on security... security came AFTERWARDS and was more like a FEATURE than a standard.
Security should be a standard... not a feature.
When Ubuntu or whatever distro finally gets a 50/50 market share with Microsoft, we WILL see attacks on Linux. Because, hackers want to attack the OS with the most usage for maximum damage.
However, Linux and Open Source comminity are ULTRA FAST at fixing the virus, spyware problems compared to Microsoft because Microsoft is Buerocratic and SLOW to respond.
Linux has antivirus software ready and waiting on Stand By called CLAM/AV. It is not in use or needed at the moment!!! But, as a previous post stated Linux users should not live in a fool's paradise meaning... Oh Linux is superior and immune to viruses.
You see... it is about who actually cares in the end. More people that do actual code FIXES care about Linux and Open Source then the 50,000+ Microsofty's at M$.
It is all about numbers and Open Source / Linux clearly has more and that means will dominate in the end.
hackers don't want to attack bill gates, they want to attack the os with the most users, and security is now a standard in windows.
and the whole linux getting 50/50 market share, it will be a really really really long time before that happens, and if it happens.