The Hijacker's Guide to biological systems: Manipulation by self‐defecting or foreign agents

Biological systems employ hierarchical regulatory networks to maintain structure and function at multiple scales, from molecules to whole organisms. This manuscript draws on evidence from various fields such as immunology, developmental biology, neuroscience and evolutionary ecology to examine how foreign agents (for instance, pathogens, parasites, trophoblasts) and self defecting agents (such as cancer cells) override these networks and exploit the host’s plasticity and regulatory flexibility, reshaping system morphology, function and behaviour. By breaching physical and informational barriers, mimicking host signals, and altering immune defences, intruders take advantage of multiscale information processing, propagating their influence across levels, from local bioelectric states to higher-order endocrine and neural circuits. Through these mechanisms, intruders disrupt homeostatic setpoints, creating periods of heightened plasticity that enable system reprogramming resulting in redirection of metabolic flows and overriding of neural, immune and endocrine signalling. These manipulation strategies rely on conserved mechanisms, transforming stable structures into flexible substrates for rearrangement, remodelling host tissues and creating niches that favour their survival. Understanding these routes of exploitation clarifies core biological principles of self-organisation and identity maintenance. Mapping how intruders induce plasticity and modify host form and function informs therapeutic strategies aimed at restoring normal tissue states, while also opening potential new paths for regenerative medicine and tissue engineering.