Key difference about viruses and malware:
Linux is designed to be routinely usable as an ordinary unprivileged user. You only need root privileges to install software or make important system configuration changes, and you can do that by supplying the necessary password just to run that one program to do the installing or changing.
Windows has always been hard to use without giving yourself administrator privileges, which you have to do by logging in as an admin-enabled user, and then everything that happens on it is run in privileged mode and it's trivial for malware to install things in the operating system where they can do the most damage and/or take control of your PC.

Yes, you still have to be careful in Linux, but usually the sphere of influence of downloaded malware is restricted only to your data files. (what do you mean, you didn't have a backup?!!!...)

Of course there are sneaky ways for clever hackers and script kiddies to get root access to a Linux box, but such activities will never be on the scale of millions of infected Windows PCs worldwide being at the beck and call of spam-generating overlords just because their owners opened a dodgy email or clicked on a link on a web page somewhere.

Anahata