Cytoglobin is a drug that is used to treat chromoviral infections and arterial aging.
In the 24th century, cytoglobin was in short supply on the Dinaali homeworld. In 2377, the Allocator of Hospital Ship 4-2 was prioritizing cytoglobin to prevent arterial aging in high-TC patients on Level Blue, while denying it to low-TC chromoviral patients on Level Red. The Doctor, who had been stolen and sold to the Dinaali by Gar, found this situation appalling, as cytoglobin that could have been used to save lives were instead being used on preventative treatments.
At first, The Doctor attempted to smuggle cytoglobin from Level Blue to Level Red. However, after his friend Tebbis died from chromovirus, he resorted to infecting the hospital administrator, Chellick, with chromovirus in order to force him to divert cytoglobin from Level Blue. (VOY: "Critical Care")