An Android trojan by the name of xHelper has been found to reinstall itself after removal. Worse still, the malware reinstalls itself even after a factory reset. Malwarebytes has devised a fairly complex manner of removing the malware, which it says is the worst mobile malware the antivirus firm has ever encountered.
https://www.notebookcheck.net/Android-trojan-xHelper-can-reinstall-itself-after-removal-and-factory-reset.453951.0.html