Viruses are not really a problem in Android, but spyware definitely is. If you are a person who doesn't worry about privacy then fine. But otherwise you need to protect yourself.
A firewall helps to control what goes in and out of your device. I use Avast Mobile Security (free) which combines a anti-virus (probably unnecessary) with firewall, anti theft and other security tools.
If you want to see some of the spyware on your device try "logging test app" (from play store, needs root). You'll see how your activity is logged and sent out.
Also check out XPrivacy
And similar threads. But, if you live on Facebook and the like, then privacy is not a concern for you and you can ignore all the above.
(all the above is not unique to Android and is also true on Apple products too)