As I am sure you well know, it is impossible to prove a negative. The closest you can come is the various security experts who monitor the Internet for malware and exploits and report what they find. So far in 7 years they have found only 1 exploit that affected non-jailbroken iPhones, and even that required that you install an app, then grant it permission to view your contacts. And as soon as this was discovered Apple removed the app. And Apple will not make statements about security, because anything they say might provide information that a malefactor could use.
There are several reasons for the high level of security in an iPhone:
- All contents are hardware encrypted all the time
- Apps are restricted to accessing their own memory space. They cannot acces the memory space of other apps, and can only access system data if the user grants permission. This is called "sandboxing". Each app can only play in its own sandbox.
- Interpreted languages are blocked on the iPhone. That is why you don't see Flash or Java (or other interpreted languages such as Python) supported. This means that Apple gets to review all code before it is approved for the iPhone
- Windows and Android do not support sandboxing, which makes them more vulnerable.
- A hacker cannot remotely hack into an iPhone, or at least none have yet, for all of their trying.
- Apple does not allow antivirus products on iPhones, but for an indirect reason: With sandboxing there is no way that an antivirus could inspect the contents of other apps.
- Antivirus products are allowed on Macs, but they are generally unnecessary because OS X has its own antivirus and firewall built in that is updated as needed.
- In theory an iPhone could get a virus or worm if there was a flaw in the code of iOS. Actually, there was such a flaw a few months ago, but it was patched before it could be exploited.
- Macs can be infected by malware, but they are much less vulnerable than Windows because the underlying operating system (NeXT Unix) is inherently secure.
- Android should be secure also, but so many people "root" their android OS that it it becomes vulnerable, just as jailbroken phones are vulnerable (see: http://thehackernews.com/2014/04/iphone-ios-Malware-targeting-apple-account.html)
