Industrial Control Systems are generally made up of two parts: the SCADA equipment which acquires data from sensors and controls industrial machines; and the software that allows human operators to control that equipment. Most of these malware incidents are the work of mundane malware that has, in most cases, reached ICS networks by accident. Around 3,000 industrial facilities around the world suffer such “accidental”” infections with non-targeted malware. But this doesn’t mean attacks with specialized (targeted) malware on ICS systems don’t exist.”
Source: https://www.bleepingcomputer.com/news/security/most-industrial-control-systems-get-infected-with-malware-by-accident/