In theory, a cell phone would be an excellent target for bad guys (especially with an open OS like Android). It's always on, people download random apps all the time, and it could be used to send out texts to others soliciting sites that would make the bad guy a lot of money. But they aren't often hacked. Leo can only attribute this to the fact that cell phone software engineers are aware of this and do more to protect users than desktop developers. Google also does a pretty good job of ridding the Google Play store of malicious apps. In fact, both Google and Apple have something called a "kill switch". If they discover that a malware program has been widely downloaded, they can reach out into user's phones and delete that program.
That being said, there are a number of decent free anti-malware programs for Android. Leo uses LookOut. Since it's free and doesn't really slow the phone down at all, there's no harm in getting it. But there's not a huge risk, and if it ever becomes something to worry more about, Leo will talk about it.