Since these phones are running essentially what is Linux, any techniques malicious people may use on Linux can be used on your phone too. Trojans, rootkits both userland and kernel, etc...

But "viruses" like you think about on Windows, not really. For one, like Endsormeans mentions, it's not a popular platform so not as targeted.

Having such old outdated software, browsers and plugins is one of our strengths in this regard (not targeted), but also a weakness since security vulnerabilities may exist without patches for our devices. Flash player for instance.