The answer is: Viruses.
Viruses are not considered to be living organisms because they lack the fundamental characteristics of life, such as the ability to reproduce independently. They are essentially genetic material enclosed in a protein coat, which requires a host cell to replicate.
While viruses can be incredibly complex, they do not have the organized structures and systems that define organs within living organisms. They rely entirely on the host cell's machinery to carry out their life cycle.
This is why viruses are often referred to as obligate intracellular parasites. They are not truly independent entities and can only exist and replicate within the context of a living host.
