Biology is about the underlying theories to cells, the growth of life, and the basic processes involved in any organism. Combined with Animals, you know the particular issues of wildlife. Combined with Medicine, you make excellent diagnoses of patients and understand the spread of disease (and protocols to contain it.) Combined with Surgery, you are well-versed in performing autopsies. You likely have Math and Technology skills. To perform complex tasks (DNA sequencing and other top-line techniques) will require Genetics.
In an ancient setting, germ theory and other advanced theories simply do not exist. In a future setting, you'll likely have to contend with alien life and bizarre circumstances.