Browsers are under siege now and can be hijacked by going to a bad site. Most of the time I can reset the browser and clear up the simpler browser hijacks.

For phones, it depends on the hijack. Some can be so invasive folk have to buy a new phone since, well, you can read it in a link to follow.

Just like a PC we always backup our stuff on the phone today. For simple phone infections a reset or a reset that clears everything is done. But there's a new nasty out there that the owner helps install. You can read about it at http://arstechnica.com/security/2015/11/new-type-of-auto-rooting-android-adware-is-nearly-impossible-to-remove/