This booklet is the result of a lengthy collaboration between the two authors. Our primary goal was to broaden our view of epidemics and their counterpart, regressions. Any health hazard or health condition can affect the number of epidemiologic cases and the people affected by them. It is not only infectious diseases that can change their importance in a population. This is true for all health threats and diseases. And this is precisely why epidemiology needs to explore the causes of such changes. During our discussions of this booklet, the world was hit by the Corona pandemic, and we concluded that we need to focus not only on communicable diseases, but on all health-related problems. We hope that readers will accept our generalized view of both the phenomena of epidemics and regressions, their definition, classification, description, and measurement of occurrence, and the assessment of such changes and their management. We want to make it clear that we need epidemiology as much as we need the contributions of bacteriology, virology, or immunology if we are to address public health threats.