A chromovirus was a pathogen, which was fatal unless treated with cytoglobin.

There were twelve cases of chromoviral infection on Level Red of the Dinaali Hospital Ship 4-2 in 2377. The Doctor, who had been sold to the Dinaali by Gar, became frustrated that the patients' treatment coefficients were insufficient for the Allocator to give them cytoglobin. Eventually, The Doctor deliberately infected the hospital's Administrator Chellick with chromovirus, forcing him to divert enough cytoglobin from Level Blue to treat all the chromoviral patients. (VOY: "Critical Care")

Early concept art by Victor Martinez for the Royal Children's Hospital identified it as "Royal Chromovirus Hospital". [1]