However, if you are running a Jailbroken device, then you can download Apps from non official sources and as this bypasses Apple’s App Store’s Approval processes, the likely hood is increased of a Worm or Virus which can infect your iOS Device.
Technically speaking, a virus is a piece of code that inserts itself into another program, whereas a worm is a standalone program in its own right; both seek to propagate themselves by hijacking messaging applications or via social engineering.
In either case, iOS’s sandbox structure should prevent the malware attack from getting access to other applications (in order to spread itself) or to the underlying operating system.
“Defence-in-Depth”
In a completely sane world, madness is the only freedom (J.G.Ballard).