From Cells to Cities: Understanding Public Health Through Entropy

The second law of thermodynamics tells us that systems naturally drift toward disorder. Left alone, gradients dissipate, structures decay, and entropy increases. A hot cup of coffee cools, a battery loses charge, and organized structures gradually break down. Yet living systems resist this drift. Cells maintain intricate chemical balances. Bodies regulate temperature and metabolism within narrow ranges. Human societies sustain complex patterns of cooperation that persist across generations.

How is this possible?

The answer begins with a simple but profound feature of life: boundaries that regulate how systems exchange energy, matter, and information with the world around them.

Living systems are not isolated. They exchange energy, matter, and information with their surroundings. But those exchanges occur across carefully maintained interfaces that distinguish what belongs to the system from what belongs to the environment. Without such boundaries, organization dissolves. A cell without a membrane cannot maintain its chemistry. A body without skin or immune defenses cannot regulate its internal state. And a society without institutions cannot coordinate the behavior of millions of individuals.

Modern theoretical biology has formalized this intuition. Neuroscientist Karl Friston, using concepts from probability theory, describes these boundaries with a “Markov blanket,” an interface that separates a system’s internal states from the external environment while allowing information to pass between them. 

Friston’s insight was to recognize that living organisms possess analogous structures. Internal states belong to the organism itself—its physiology, neural activity, and internal beliefs about the world. External states belong to the environment. The two never interact directly. Instead they communicate through the blanket. Sensory states convey information from the environment to the organism, while active states allow the organism to influence its surroundings.

Between these flows lies the blanket that keeps the system coherent while allowing it to remain open to the world.

Friston’s broader proposal—the free energy principle—suggests that living systems persist by minimizing surprise. More precisely, they act to minimize the gap between their expectations about the world and what they actually encounter. A thermostat provides a simple analogy: it has a target state and acts whenever reality diverges from it. Similarly, organisms keep themselves within a narrow range of viable states by predicting their environment and acting to maintain stability.

Just as cells sustain membranes and organisms maintain sensory interfaces, human societies build boundaries that stabilize collective life.

Openness and Fragility in Complex Societies

Societies thrive on openness. Just as living systems exchange energy and information across their boundaries, human communities depend on the movement of people, goods, and ideas. People move freely within cities and across borders. Goods are transported through global supply chains. Information flows via digital networks. Openness drives prosperity, creativity, and cultural exchange.

But openness also creates pathways for disruption.

Respiratory viruses spread through shared air. Financial shocks propagate through interconnected markets. Misinformation cascades across social platforms. In these moments, what is threatened is not merely the stability of individuals but the coherence of the larger system—the social organism itself.

A society that is too open risks losing its capacity for coordination. But a society that becomes too closed risks something equally dangerous: rigidity. Excessive insulation suppresses innovation and weakens the adaptive capacity needed to respond to new threats.

The central challenge for any complex system, biological or social, is therefore not simply choosing between freedom and control. It is finding the dynamic balance between openness and order.

Public health functions precisely at this boundary. Its role is to regulate how much permeability a society can tolerate at any given moment. How easily risks, pathogens, and behaviors spread through the population, and when reinforcement of boundaries is necessary to maintain stability.

When viewed through this lens, many public health conflicts become disputes about something deeper than policy. They become disagreements about where the boundary of the self should be drawn.

Public Health as a Societal Markov Blanket

Under ordinary circumstances, the boundary between individuals and society is relatively loose. Individuals make personal decisions about their health—whether to vaccinate, seek medical care, or adopt preventive habits—while public health institutions operate at the population level, monitoring disease patterns and recommending interventions.

Information flows from the collective to individuals through advisory systems, clinicians, and media. Individual behaviors flow back into the population through actions such as vaccination, hygiene practices, and social interaction. These informational and behavioral exchanges form the functional interface between personal choice and collective health.

In stable periods, the connection between individual actions and population-level consequences is not always visible. Most people rarely encounter vaccine-preventable diseases, and the effects of public health policies are often indirect or delayed. As a result, the perceived need for strong coordination between individuals and public institutions remains relatively low. The societal Markov blanket remains present but relatively permissive.

Pandemics alter this relationship dramatically.

Respiratory pathogens ignore the boundaries we imagine around our bodies. A virus does not respect personal autonomy. It exploits the very openness that makes social life possible—shared air, shared surfaces, shared space.

When a novel infectious disease spreads rapidly, the statistical coupling between individuals and the population intensifies. Individual behaviors—whether people isolate, wear masks, or accept vaccination—begin to have immediate and visible consequences for others. At the same time, the population-level state of the epidemic directly affects individual risk.

In this situation, the boundary between autonomy and collective health tightens. Public health messaging intensifies, surveillance becomes more visible, and policy interventions such as vaccination campaigns, quarantine measures, and mask mandates become more common.

From a systems perspective, high vaccination coverage constrains the number of possible outbreak trajectories. It reduces the variability of the epidemiological landscape. In thermodynamic language, it lowers entropy within the transmission network.

People often accept these changes during a crisis because the signals of danger are clear. Rising case counts, hospitalizations, and personal experiences with illness provide powerful feedback.

In epidemiological terms, the relevant “organism” during an outbreak is not the isolated individual but the network of interacting individuals.

As the crisis subsides, however, the coupling loosens again. Without visible signals of risk, individuals update their internal models toward the belief that the danger has largely passed. Vaccination programs are uniquely vulnerable to this dynamic because their success removes the very evidence of disease they were designed to prevent.

If enough individuals decouple their decisions from population-level dynamics, herd immunity weakens and outbreaks reappear. The system oscillates between periods of stability and renewed vulnerability.

Metabolic Health: A Different Entropic Landscape

Diet and metabolic health operate in a very different system.

The harms of poor diet—insulin resistance, cardiovascular disease, fatty liver—typically unfold slowly and internally. They do not spread from person to person the way respiratory viruses do.

Although food environments shape population health, the immediate spillover of my dietary choice on your health is weaker than in infectious disease.

The relevant organism here is closer to an individual embedded within an environment rather than a tightly coupled network.

Metabolic disorders develop over long timescales. The signals linking cause and effect are delayed and diffuse. A sugary drink does not cause a heart attack tomorrow. It nudges a metabolic system along a trajectory that may culminate decades later.

Public health responses, therefore, focus less on mandates and more on environmental recalibration—policies affecting food marketing, agricultural subsidies, labeling, school nutrition, and urban design.

These interventions aim to reshape the environment so individuals can maintain metabolic stability more easily.

Politics and the Boundary of the Self

Political movements interpret these boundaries differently. The conflict arises because two different boundaries of the self are being invoked. One treats the individual body as the relevant system. The other recognizes that during an epidemic the relevant system is the network of interacting bodies.

With vaccination, the interface between individual behavior and collective outcomes is extremely tight. Infectious diseases propagate through networks of human contact, so one person’s vaccination decision directly influences the risk faced by others. Public health responses are designed to coordinate behavior across the population.

For political traditions that emphasize individual autonomy and skepticism toward centralized authority, such coupling can appear as institutional overreach. The interventions are visible, often mandated, and associated with government agencies or expert bodies. Political narratives may therefore frame vaccination policies as infringements on personal autonomy rather than as mechanisms that stabilize a biological network.

Metabolic disease presents a different structure.

Although obesity and chronic illness carry enormous population-level consequences—healthcare costs, lost productivity, shortened life expectancy—the immediate causal chain appears more individualized. Dietary habits unfold over long timescales and do not spread directly between people.

Because the boundary appears looser, political narratives around obesity more easily emphasize personal agency and environmental reform rather than collective behavioral mandates.

Critiques of the modern food system—ultra-processed foods, agricultural subsidies, aggressive marketing, and sedentary built environments—can be framed as correcting distortions in the external environment rather than regulating individual bodies.

In debates over food and chronic disease, responsibility can be placed on structural features of the environment—industrial food production, pharmaceutical dependence, regulatory capture—rather than on centralized behavioral mandates.

Another difference lies in the visibility of risk signals.

Infectious disease threats fluctuate rapidly. When a pandemic subsides, the signals that justified strong collective action fade. 

Metabolic disease, by contrast, remains continuously visible. Rising rates of diabetes, cardiovascular disease, and obesity unfold across decades, reinforcing the perception that the problem is persistent and structural rather than episodic.

Seen through the lens of thermodynamics and systems theory, both vaccination and obesity represent attempts to stabilize the system against different forms of disorder. The differing political responses therefore reflect not only ideology but also the structure of the underlying biological systems.

Managing Entropy in Public Health

The challenge is not to eliminate entropy, which is impossible. The second law guarantees that disorder will always press against the boundaries that living systems build. The task, instead, is to manage that pressure intelligently.

Entropy teaches humility. Order is always temporary. Pathogens evolve. Food systems drift toward profit-maximizing configurations that may not align with metabolic health. Social trust erodes if neglected.

In public health, sometimes the relevant organism is the individual body, and other times it is the entire network of interacting people. The appropriate boundary of the self shifts depending on how tightly our biological and behavioral systems are connected.

Living systems persist not by sealing themselves off from the world but by managing their openness wisely. Cells regulate what crosses their membranes. Organisms interpret signals through sensory interfaces. Societies construct institutions that coordinate millions of individual actions. Survival depends on maintaining boundaries that are neither too rigid nor too porous, keeping entropy at bay while remaining open enough to adapt to a changing world. Public health, at its best, does the same.