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).