Introduction

Virulence of pathogens, the origin and growth of neoplasias, and their treatment by chemotherapy or vaccination, are problems involving many complex processes on different organizational levels of the biological system. In recent years molecular biology has made an important step forward in identifying major elements in these processes. Yet, it becomes increasingly clear that in many cases prognosis is determined by the dynamic interaction between elements of the system, rather than by their presence or absence.

Today, thanks to the efforts of Anderson, Dietz, Hethcote, May and others, it is not a strange idea to employ population dynamics theory for identifying optimal vaccination strategies for human populations. In contrast, drug protocols for individual patients are still determined by trial and error.

The present paper attempts to show how a theory of population dynamics in perturbed environments proves useful for studying disease processes across several organizational levels of the biological system. A method for increasing selectivity of AZT treatment of HIV infected individuals, and a method for improving measles vaccination policy will be described, both being motivated by the same general theory.

The relation between the population and the environmental periodicities

Until recently, the approach to population dynamics was governed by the concept of equilibrium, and environmental disturbances, although much alluded to, were seldom incorporated in the analysis of life-history strategies. In contrast to the predominant view, my work investigates population dynamics over a wide spectrum of time-scales for the environmental perturbation.