First of all, this question shows a lack of research. Yes, there is some misconception of unix systems like Linux and Mac "not getting virus's". Some people think that's because just there's more Windows systems out there to target. However, if you consider that probably 90% of all the servers and embedded devices on Earth run some form of Linux, than it's clear there's plenty of Unix/Linux systems to target as well. Servers also often have valuable information on them, fast internet connections, and are on 24/7, and thus are valuable targets. And while Linux systems totally are targeted and compromised all of the time, I'd bet that the percentage of compromised Windows machines is much higher for a couple reasons. First, as mentioned, a Unix system is just easier to secure, mainly because of the way permissions are handled. The second factor, in my opinion, is that people running Linux are typically more informed, and thus security aware. System compromise happens because the user either did something they should not have, or didn't do something they should have. Android is currently the most popular OS in the world, so the percentage of security aware users is inevitably going to be lower.
Often times in the Linux world, compromise happens because of lazy IT technicians, and is not the system's fault. Most of the time, these incidents totally could have been prevented, take for instance the recent US government hacks. With Windows and Android, it's a totally different story. The Windows community doesn't have hundreds of thousands of eyes scrutinizing it's source code all of the time, so bugs and vulnerabilities go overlooked and Android, being an embedded system, does not get kernel updates and such as often as desktop systems. In a proprietary world, you also have to wait for the developer to issue a patch because nobody has the source code. So, when a vulnerability is found, it can be like open season till Microsoft or whoever patches it.
Because Linux is free for anyone to use, it's often used to power embedded devices.Android is indeed a unique situation, because it's driven by a Linux kernel, and while it does have a pretty good permissions system that could help to prevent the spread of trojans and viruses, that system has been completely perverted. Apps tend to ask for way more permissions than they need, and people tend to do things like 'Sure, I'll give this flashlight permission to access the internet and read my location, why not?'
Android is also a high value target, as cell phones tend to carry more personal information than personal computers ever did, after all we have them with us all of the time, pay for stuff with them, do our banking on them, etc. Also, while it's technically Linux powering the Android system, the actual apps running in userland are written in Java, which is something else entirely.
Finally, the reason why we don't have gigantic anti-virus databases filled with signatures of every bad thing that ever was is because (wouldn't you know it), it turns out that's a hell of a lot easier to just white-list the stuff that should be allowed to run on your Linux machine than it would be to try to identify and enumerate all of the badness in cyberspace and than rely on software to blacklist it. AV is a counter productive (and expensive) approach to the problem. This is just an opinion, but the billion dollar market for anti virus software is likely part of the reason there's so many viruses out there. It's business.
There is stuff like chkrootkit and rkhunter out there, but most of the time all it will give you is false positives. With Linux, it's much easier to see what's happening on your system, so there is not as much need for something like AV. Most of the few programs that do scan for bad things are looking for rootkits. Ironically, the rest are actually looking for Windows viruses, for instance mail servers may run clamav to make sure Windows or Android clients don't get infected by something they transmit. It's also worth mentioning that cell phones have always been extremely insecure to begin with. GSM has been broken since it was invented, the feds now want Apple to put an 'encryption backdoor' in the iphone, and basebands are remotely manipulated through ISMI catchers by criminals an police alike. None of that is Linux's fault. Like others have said, the system is only as secure as the weakest link.