viruses have to be programmed to work on the OS
A windows virus will not work on an iphone. an iphone is like a Mac. It uses a different method of running programs.
but assume the virus is programmed to work on an iphone and assume it gets past the review process
since no app can run in the background you would have to have the app open for the virus to run. close the app and the virus stops
but assume you jailbroke your iphone and let the app run in the background. well, then you brought the problem on yourself
Your blackberry runs Java which does run in windows. a virus could be programmed to run on a blackberry AND windows. I believe blackberries prompt you before they will install and run anything, however