Immunes

Ancient Rome's military was highly advanced for its time, divided into multiple units to maximize efficiency and power. One such unit was the immune class. By definition, the immunes were legionary soldiers who possessed specialized skills, qualifying them to perform duties atypical of a Roman soldier. Immunes’ expertise in their respective fields allowed them to be exempt from the more tedious and dangerous tasks other soldiers were required to do, such as ditch digging and rampart patrol.

Becoming an Immune
Prior to becoming an immune, men were required to serve as milites (also known as munifex), a non-specialist regular soldier. These men were the soldiers that made up the bulk of the legions, liable to perform guard duties, labour work and other less than desired duties. Milites would usually have to serve for several years before becoming eligible for training to become immunes.

Immune status within the army was achieved either through selection or through promotion. If not possessing the specialist skills that could see a soldier chosen to become an immune, the legionary who wished to become one would have to undergo a period of specialist training, during which time they would be known as discens. The discens received the same basic pay and board as the non-specialists until he qualified for immune status.

Typical duties
Engineers, artillerymen, musicians, drill and weapons instructors, military police, carpenters, hunters and medical staff were among the multiple specialized jobs immunes provided for the Roman army. Immunes also received better pay than the regular troops.