There are scanners and tools. But it's not worth the hassle to develop them more.
No system is immune to viruses, trojans, malware. In the end it's just some piece of code that runs, but doing things you don't want. It happens in every systems and 99% of the world is probably vulnerable at some point.
Android has become a really interesting target, this explains why you can see some media activity about vulnerabilities targeting android systems. But it's really nothing special, and the best defense is as always stay up to date, do backups and be alert of suspicious programs/links/activities.
I have been running windows 7 for 2 years without an anti-virus. My system is fast and I never had any problems. It's easy if you know where to look, I know exactly what's executed, and why. If I don't know, I look for it, and I may remove it from my system.
On Unix it's way easier to monitor a system and find suspicious programs/activity. Most people using Unix are competent (more than windows uers) so malware writers will not change their targets that easily, because less chances of success.
So yes, virus are potentially everywhere. But using some diagnostic tools will probably reveal them. On a noisy system as windows and android, viruses' noise will be hard to spot, and at this point it's realistic to consider that you are already infected. Never think you are safe except if you are sure you know what is going on. It takes years of studies and experience to be a competent system administrator, if you install an Unix system, it's your responsibility to become administrator. Or else, accept the fact that you'll never be safe, especially against targeted attacks.
Now let's talk about rootkits. This is the next level, if you get one, you will probably be specifically targeted by someone very competent. As most working rootkits are very recent and kept hidden. You will probably never notice it. If you're lucky your system will crash and this may alert you. There have been rootkits targeting Unix for years, Mac OS X, FreeBSD (There's a book to learn how to write them, very fun, Designing BSD Rootkits), GNU/Linux, etc.