This is the 3075th most frequent Russian word.
инфекционной
Infectious or pertaining to infection.
Here, "инфекционной" is the adjective form of the word "инфекционный" (infectious) and modifies "болезни" (disease), describing its nature as related to infection.
Эпидемия инфекционной болезни распространилась быстро.
The epidemic of an infectious disease spread quickly.
In this sentence, "инфекционной" describes "медицины" (medicine), indicating the specialization of the department as dealing with infectious conditions.
Она работает в отделении инфекционной медицины.
She works in the infectious medicine department.
Here, "инфекционной" characterizes "патологии" (pathology), showing the specific focus of the study material on infectious aspects.