Kaspersky Labs wrote an informative blog post on why you don’t need an iOS antivirus app. In short, apps can’t interact with other apps’ data in a sandbox environment.
[Apple Updates App Store to Remove iPhone Antivirus Apps]
iOS Antivirus App
Apple designed iOS from the ground up with security in mind, and doesn’t allow any iOS antivirus app into the App Store, simply because they aren’t needed. Every iOS app is kept inside a sandbox. This means that a malicious app won’t be able to secretly steal data or infect other apps.
Additionally, you can’t install an app outside of the App Store (on a non-jailbroken device). There is a reason why Android users get so much malware, and it’s partly due to the fact that you can install apps outside of the Google Play Store via APK files.
Apple has a good review process (usually) and examines the code of all apps before they are allowed into the App Store. It’s difficult for a developer to sneak a malicious app onto the platform.
Although there are plenty of security apps in the App Store, none of them are antivirus apps. An antivirus program works by scanning your device’s filesystem for malicious code. But because every app is sandboxed, an iOS antivirus app wouldn’t be able to scan other apps, or iOS itself.