You don’t need an antivirus for your iphone. Apple’s iOS operating system takes care of the security of your iphone for you. It is actually a security issue if a 3rd party app is allowed to snoop around your phone and access you personal documents and pictures. The way Apple has designed its phone, it largely limits the way these apps access data.
iPhones are designed with security in mind. Although the security is not always foolproof, especially when there are new risks emerging everyday, Apple does try their best to keep ahead and update their operating system to fix any security issues.