[from link] Jones has written about the origins of the term “wave” when referring to a disease outbreak, noting it arose as an analogy in the 19th century when science was beginning to apply the notion of a “wave” to many concepts and situations.

There is no rule in epidemiology that predicts a diseases's "second wave" will always be worse than the first. In many instances, including the current COVID-19 pandemic, the concept of waves may not accurately describe the course of the pandemic. Rather, the trajectory of the virus will largely be determined by the varied human responses to the new coronavirus.