Trojans and viruses are two different monsters altogether. Viruses are designed to do damage to the file system of the host computer. Trojans on the other hand are small packets of data hidden inside legitimate programs and apps and are designed to sniff out things like passwords, credit card numbers, and contacts.
Then send them to a recipient over the web.

Last year security experts (I hate that word) trawled the app stores pulling random apps from the shelves and scrutinizing them. They found literally dozens of Android and iPhone apps that contained data that allowed the apps to report back with user info without the end user knowing or being informed prior to installation. There was maybe a small number of other apps for other O/S'es involved but they were probably less of a significance.

In this day and age it seems the big players are the OS manufacturers, service providers and data mining companies are the main culprits nowadays.

Have a lookee here: http://www.badphorm.co.uk/page.php?2

http://www.guardian.co.uk/business/phorm