There seems , and still seems to be a great deal of confusion and different views on this.
So lets just clear up what can be REALLY said :-
A virus is a bit of “code” that can often auto infect a system, replicate , infect and spread — it usually requires an infect email, attachment, Webapge accessed , Then it gets busy without needing the user to respond to any alerts!
A Trojan is a program or App that is “pretending” to be something nice / useful etc .. when in fact it is not, now a Trojan NEEDS you the user to start off the installing of it / downloading it from website etc
Now, currently, there is no virus for any iOS device — but could be a few Trojans out there BUT apple is quite strict as to what actually gets to the App Store.
Android — Avast has reported a virus recently ( that does not need the device to be rooted ) , And apparently Google is not as strict on their app store.
BUT downloading app be it from the App store with apple or Google play on Android , that is the BEST way to stay safe, Jail breaking your iOS device or “rooting” your Droid whilst opens the doors to a host of maybe cool stuff, also deactivates the devices stock security , So in a word ( unless you REALLY know what you are doing ) DO NOT jailbreak or root your device!
That all said now, On the Droid market there are a host of anti virus apps, like Avast IF you really need one, download it ( from google play! ) and monitor how battery / performance is, If its not great then uninstall it, And try another!
Now Intego has an iPhone / iPad app — however the reviews stink! and i would possibly avoid it at all costs!
Its not free, and you have to pay a yearly ( yet small ) fee .. But again looking at the reviews it seems a pointless App particularly if you have not Jailbroken your iOS device!