Zero day malware is malware that is not yet known to anti-virus software application suppliers or business. It makes use of vulnerabilities in software or hardware that have not yet been covered or fixed. The term "no day" comes from the fact that as soon as the malware is discovered, there is zero time to patch the vulnerabilities prior to aggressors can exploit them.
Absolutely no day malware is often used in targeted attacks versus particular companies or people. The attackers will thoroughly choose their targets, and then craft their malware to specifically exploit the vulnerabilities in those target's systems. This makes zero day malware very tough to protect and detect versus.
One of the most popular examples of absolutely no day malware is the Stuxnet worm, which was used to attack commercial control systems. Stuxnet had the ability to spread itself through USB drives and infected networks of computer systems, without being discovered by anti-virus software. https://bikramyogauk.net/?p=1525 It then specifically targeted Siemens industrial control systems, and caused physical damage to centrifuges in Iran.
No day malware is a major danger to companies and people. It is necessary to keep your systems updated with the most recent security patches, and to be familiar with the most current hazards.