A Diagnosan without a mask
Diagnosans were a species from the Uncharted Territories with a natural ability detect and treat a variety of ailments in all species -- including non-humanoid species such as Leviathan biomechanoids. Because of these abilities, many of them became doctors and physicians and their services were highly sought after. Some diagnosans earned renown by inventing medical technology, such as cryogenic tubes and coolant suits. All diagnosans were gentle, soft-spoken creatures.
The language of Diagnosans was incredibly complex, matching the complexity of their brains. Because of this, translator microbes were incapable of translating it and it took over two cycles of careful study for it to be understood until it was eventually programmed into the Federation's universal translator technology.
An example of a Diagnosan in a mask
Ironically, diagnosans were extremely vulnerable to infection within their own bodies. In order to prevent coming into contact with contagions, diagnosans typically wore masks over their faces whenever possible. They would remove these masks only within a sterile environment, usually while performing surgery in a sterilizing force field. They would need to put their masks back on before exiting or turning off the force field, as even the slightest trace of a disease could prove fatal for them.
Notable Diagnosans[]
Source[]
Diagnosans are seen in Farscape.