Experimental data were summarized to assume that dinitrosyl iron complexes (DNICs) with thiol-containing ligands are an endogenous "working form" of the nitric oxide (NO) system in living organisms. DNICs can function as donors of both neutral NO molecules, which are responsible for positive regulatory effects of the NO system on various physiological and biochemical processes in humans and animals, and nitrosonium cations (NO⁺), which are responsible mostly for negative cytotoxic activity of the system. Special attention is paid to the finding that DNICs, especially in combination with dithiocarbamate derivatives, suppress SARS-CoV-2 infection in Syrian hamsters.