In the five years since the first iPhone was released, there has never been a serious known case of iOS malware on an non-jailbroken device.
Virtually every version of iOS has been quickly jailbroken (that is, modified to allow installation of apps and hacks not authorized by Apple or the mobile carrier).
Jailbreaking is accomplished by exploiting security vulnerabilities in iOS. The same exploits used to jailbreak (an arguably legitimate hack) could just as easily be used to infect an iOS device with malware.
Read more to know the possible dangers and be aware: