While it may be true that most virus's are made for windows machines, it not accurate to say this is the reason why Linux is more secure. If Linux was used by 100% of people, it would still not be vulnerable to online virus attacks. Why?

1. The linux kernel and outside processes are separate. this means that if you do get a virus, it can't do much damage since it has no access to the kernel.

2. As long as you don't stay logged in as root, the virus won't be able to do anything, as it would need root permissions to do damage.

3. Most vulnerable file systems in linux such as /var (this holds logs, and file caches) and /tmp (this holds temporary files) are marked "non-executable" which means nothing can be executed from them. which means the virus can't do anything. (I have these directories along with /usr and /home in their own partitions which adds double security)

The only thing you really need to worry about is someone physically getting into your computer, which can happen through ssh, imap or any open internet port. Just make sure you have a firewall running, and you can block everything except those services you use. The firewall will also list the ip and service connected to your computer, so if you see something fishy, you can block it.

ps: you shouldn't need to run an anti-virus scan at all. The only reason to do so is to keep your outgoing mails (if you use thunderbird) clean for your windows using friends and to clean up any random windows executables sitting around. (these can't do anything to your system, they just take up space on the harddrive.)