Manipulation of biological systems by self-defecting and foreign agents

Biological systems maintain structure and function through hierarchical control, redundancy and surveillance across scales. These same features create leverage points for agents that establish a persistent presence in host systems, including foreign agents such as pathogens, parasites, trophoblasts and gall-inducing insects, and self-defecting agents such as cancer. This review synthesises evidence across immunology, developmental biology, neuroscience, oncology and ecology to treat manipulation as a staged control problem in which local access is converted into distributed effects through shared signalling and feedback loops across scales. We outline a six-step framework in which manipulators establish access, interfere with boundary and identity checks, exploit transient plasticity to reset homeostatic constraints, redirect immune, endocrine and neural communication, remodel host structure through developmental programmes, and stabilise altered states through niche construction and epigenetic remodelling. The framework separates mechanisms required for initiation from those required for maintenance and long-term persistence. We illustrate this framework through four case studies: trophoblast invasion in pregnancy, tumour progression, parasitoid manipulation by Cotesia congregata, and plant galls. The synthesis highlights recurrent vulnerabilities in boundaries, shared signalling and plasticity windows, and motivates stage-specific experiments that test whether candidate mechanisms drive entry, sustain the manipulated state, or increase resistance to reversal. The same logic can help organise thinking about complex pathology in which altered states are maintained by distributed feedback.