The adverb commonly is good for talking about something that usually or ordinarily happens. Mice, for example, are commonly afraid of cats.
Commonly describes an action that's to be expected. You could say that the bus commonly picks you up fifteen minutes late, or that colds and flu are commonly passed around between kids at school. The earliest use of commonly, around 1300, was to mean "in a way common to all," from the adjective common, or "belonging to all." The Latin root communis means "in common, public, or shared by all or many."