# A Conceptual Disease Cycle Model to Link the Size of Past and Future Epidemics

## TL;DR

This paper introduces a model showing how past and future epidemics are connected through evolutionary and environmental changes.

## Contribution

The novel contribution is the Disease Cycle model, which integrates evolutionary feedbacks and environmental change into epidemic dynamics.

## Key findings

- Epidemics influence natural selection and shape host-parasite diversity.
- Genetic diversity in host or parasite populations affects future epidemic severity.
- The Disease Cycle model supports a holistic view of epidemics as coevolutionary outcomes.

## Abstract

Populations of humans, animals, and plants face ongoing threats from infectious disease epidemics. While host–parasite coevolution plays a central role in shaping these dynamics, epidemics are often studied in isolation. I propose a simple “Disease Cycle” model that connects past and future epidemic sizes within the context of environmental change and evolutionary feedbacks. Drawing on recent evo‐epidemiological research, I highlight three key themes: (i) how epidemics influence the strength and direction of natural selection, (ii) how host and parasite diversity shift through evolving resistance and infectivity, and (iii) how genetic diversity in either population may affect future epidemic severity. Although gaps remain, current evidence supports this integrative model. Future research should explore how the Disease Cycle applies to non‐model organisms with low coevolutionary potential. This framework encourages a more holistic view of epidemics as dynamic outcomes of host–parasite coevolution.

This article introduces a “Disease Cycle” model linking past and future epidemic sizes through evolutionary and environmental feedbacks. It emphasizes how epidemics drive natural selection, shape host–parasite diversity, and influence future disease severity. The framework offers a holistic perspective on epidemics as dynamic products of coevolution.