It is true for viruses:

Explanation

Viruses are tiny infectious agents that lack many of the key characteristics of living things. They cannot reproduce on their own, as they must infect a host cell to multiply. They have no metabolism and cannot produce energy or synthesize proteins. Unlike cells, viruses lack cellular structure and consist only of nucleic acids (either DNA or RNA, never both) enclosed in a protein coat. They are the smallest infectious agents, visible only under an electron microscope, and rely entirely on host cells for replication.