As these devices are connected to the internet, there are high chances for virus infection. If infected, your passwords, bank details, and other personal information can be stolen.
Lookout is a free mobile antivirus software for Smart Phones (Android, Blackberry, Windows mobile) that keep your mobile device safe from viruses, malware and hackers.
Besides the antivirus protection, the program has other two features. It can locate missing phone and back up your contacts, photos and other data.
More information and download Lookout at source.