iOS sandboxing is extremely secure. Because iOS is not open-sourced like Android, the only way to infect an iOS device is one that's custom made for that particular device. Meaning if you want to infect every single iOS device that's out there, you'll have to create a good 900 million (approximately the amount of iOS units to be sold from the start of the iOS branch up to today) unique viruses to infect each and every one of them, which is unlike Windows where you can create one and infect the entirety of the globe.
Or, the hacker will have to have a hardwire access to your device.