You can't get viruses on your iPad, I'm on mine right now.
That's like saying it's impossible to hack Blockland. Oh wait, idiots do claim that.
There's a vast difference between being unable to get viruses on something and viruses for something existing.
IE: Macs were never immune to viruses, they just weren't a main competitor to Windows for a long time so they were off the radar. Once Macs become mainstream and viruses start coming out for them, SURPRISE! Macs turn out to be far MORE vulnerable, and Apple has to rush out AV software in a hurry.