A secondary infection was a term used to describe an infection enabled by a weakened immune system that had already been compromised by a prior infection.
In 2369, Doctor Julian Bashir administered corophizine to lower the chances of Chief Miles O'Brien acquiring a secondary infection other than the aphasia virus. (DS9: "Babel")
In 2377, Tebbis died due to a secondary infection of acute chromoviral disease after his medication allotments had been used up. (VOY: "Critical Care")