Let me put it this way.
Only 0.7% of all mobile device malware programs are written to target the iOS platform (that's only 1 out of every 143). And the only way any of those malware programs can get onto an iOS device is either by:
A) The iOS device has been 'jailbroken' (i.e., someone has used another piece of hardware to manipulate and change the software of the iOS device)
B) Direct, physical contact (i.e., someone has possession of your device and installs it manually, which actually requires 'jailbreaking' it, at least temporarily.
If you have not jailbroken your device, and no one else has had possession of your device, unattended, then you have zero cause for alarm.