It's difficult to argue that iOS is not a secure platform. At the same time, Don't make the mistake of assuming that the iOS platform and Apple's App Store are invulnerable to attack. They're not. But they are more secure than the Android equivalents.
Apple designed the iOS so that every app runs in its own, restricted space. Apps have limited abilities to communicate with each other, so its pretty tough for the virus to easily spread in IOS device.