No operating system is totally safe from malware attacks, but some have a broader risk exposure than others. A sure-fire way to have an operating system become popular with hackers who want to have a little fun is for the operating system to gain popularity with “normal” people. The Android operating system has certainly reached that point. According to one study (http://thenextweb.com/google/2012/10/25/in-one-year-android-malware-up-580-23-of-the-top-500-on-google-play-deemed-high-risk/), the number of Android apps that contain malware rose 580% last year. With this in mind, it is wise to consider software for your phone that will help mitigate the risk.
I chose to use avast! Mobile Security on my Republic Wireless phone for a few reasons. First, it is free. Second, it is lightweight and only causes a very small impact on total system performance; most performance penalty is when downloading/installing software. Lastly, in a relatively recent study (http://www.droid-life.com/2012/03/07/over-40-anti-virus-apps-tested-only-7-have-malware-detection-rate-of-over-90/), avast! Mobile Security was able to detect the most malware risks of any scanner available on the Play Store today. Oh, and another cool feature is that avast! Mobile Security can help you find your phone if you misplace it or someone helps themselves to it.
*Full disclosure: I have no affiliation with AVAST Software and only offer this as a recommendation that is based on much research and experimentation on my part.