When you're sitting on top, you have a great view of others. The downside is, you're now easy to spot and make a better and more tempting target.
As Android's popularity has risen*, so has its attractiveness to hackers. This is akin to underdog Firefox becoming the favored alternative to Internet Explorer when the latter was under seige, and then the hackers turning their sights on the more visible Firefox.
So the bad news for users of Android is that it's now under attack. One way you could be affected is by downloading rogue apps from third-party websites, such as recounted in the August 13, 2013 story New Android malware is being distributed through mobile ad networks.
The good news is, if you read the article closely, you'll notice that the mobile ad networks it mentions are more common in areas where mobile devices can't access the official Google Play store or users have difficulties in purchasing applications in a legitimate manner. According to Antone Gonsalves in his September 27, 2013 article Become a hacker. Coding experience not needed., this is generally in places like "Asia, Eastern Europe and Russia (where) infection rates for Android smartphones are higher because people regularly download apps from sketchy sites. In the U.S., the vast majority of people use Google Play, so the chance of infection is minuscule."
So even though you Android users might breathe a little easier seeing this, note the importance of using Google Play as your marketplace for apps. Since a few bad ones slip through occasionally, it's also a good practice to read the apps reviews and download statistics before clicking that install button.
And for a nice rundown of Android antivirus software, see Darlene Storm's article Mobile malware madness: Favorite target? Android. Here's 3 free security apps. It paints a less rosy picture, but then it is from the point of view of AV vendors. Still some good advice at zero cost to you.
In summary, nothing is safe 100% of the time but you can take some precautions to protect yourself: download only legitimate apps, run an antivirus program, and use your common sense. It something appears a bit iffy, steer clear.
* According to a survey from the Pew Internet & American Life Project, in May 2013, Android lead iOS by 3 percentage points (28% of mobile phone owners' smartphones were Android, 25% running iOS). Read more about smartphone trends at US Smartphone OS Race Still Close, as Men, Younger Users Favor Android.