superinfection
Medical term: a new infection occurring on top of an existing one, often with a different pathogen or a resistant strain.
Superinfection can occur after antibiotic use.
coinfection
Medical term: simultaneous infection by two or more pathogens.
Coinfection with HIV and tuberculosis is a serious problem.
Compound of 重複 (overlap, duplication) and 感染 (infection). The exact historical derivation is uncertain; the term is a modern medical coinage.