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“Atelectasis pulmonum” means incomplete expansion or collapse of lung air spaces; in the newborn it classically describes portions that have not expanded adequately after birth. Modern atelectasis also includes collapse of previously aerated lung. The older distinction between primary non-expansion and secondary collapse is useful when reading this paper.

The affected lung is airless, often dark and firm, with reduced volume, a depressed surface and little or no crepitation. Failure of inspiratory effort, obstruction of a bronchus by secretions, and external compression are different routes to this appearance. Modern physiology adds the role of pulmonary surfactant in opposing alveolar collapse; deficiency in premature infants belongs to neonatal respiratory distress syndrome, not an explanation the 1867 question itself supplies.^[ref]

The main historical confusion was with pneumonia. A pre-1867 review of Charles West's work distinguishes primary non-expansion from secondary collapse, and contrasts both with pneumonic hepatization. A collapsed region loses air and volume; inflammatory pulmonary consolidation fills air spaces with exudate. Both can look solid.^[ref]

The classical experiment introduces air through the trachea or an appropriate bronchus while the affected region is observed. Uncomplicated collapsed lobules can expand and regain buoyancy; inflation does not simply remove inflammatory exudate. The historical review describes this distinction.^[ref] The experiment's interpretation requires more than one observation: whether a region takes air, whether its volume changes, and whether its contents show inflammation are different measurements. Cut-surface inspection and histology test the last point directly. Floating or sinking alone cannot distinguish all mixtures of collapse, fluid and pneumonia, and cannot reliably establish whether an infant was born alive.

Other potential confusions include pulmonary oedema, pulmonary haemorrhage, and compressed lung beside a pleural effusion or pneumothorax. Fluid or blood in the air spaces differs from collapse alone, although the conditions can coexist; external compression can actually cause atelectasis. Pulmonary hypoplasia is different again: an underdeveloped organ is not merely an otherwise normally formed organ awaiting inflation.^[ref]

Reduced aeration is the observation; non-expansion, obstruction, compression and inflammatory consolidation are competing or coexisting explanations. Gross inspection, controlled inflation and histology help discriminate them.

An 1867 answer would distinguish general depletion to relieve a supposed dangerous circulatory overload from local depletion intended to relieve a circumscribed inflammatory condition. General bloodletting meant opening a vein by venesection. Local bloodletting meant medicinal leeches or wet cupping; ordinary dry cupping did not remove blood. In wet cupping, superficial incisions and a partially evacuated cup withdrew blood from the skin. The apparatus and the quantity removed mattered: a supposedly local procedure could still produce substantial systemic blood loss.

In the period's vocabulary, a robust person with an early “sthenic” attack, a strong full pulse, severe oppression of breathing and an apparently engorged lung might be considered for general bloodletting. The intention was to lessen vascular distension and the work imposed on the heart, or to arrest an advancing inflammation. Local bloodletting over a painful region was intended to moderate local inflammation without the full depletion of venesection. These were therapeutic theories, not proof that blood beneath the skin drained an inflamed organ selectively. A contemporary account of pneumonia explicitly preferred early bloodletting in severe “sthenic” cases and distinguished them from debilitated “asthenic” cases.^[ref]

Even within that framework, the decision needed the stage of illness, previous blood loss, age, general strength and response to any previous depletion. A weak rapid pulse, pallor, faintness or exhausted convalescence argued against further bloodletting. “There is fever” or “the blood looks inflammatory” was not an adequate indication. A hot flushed face alone could not establish excess circulating volume. The period evidence also needs scrutiny: selecting the most severe cases for bloodletting, then comparing their recovery with mild unbled cases, confounds treatment with initial severity. The reported recoveries establish what was done; they do not isolate its causal effect.^[ref]

Modern correction: remove blood only for a defined indication, not to treat ordinary fever, pneumonia or a presumed surplus of “inflammatory” blood. In haemochromatosis, controlled venesection removes iron contained in red blood cells; repeated replacement of those red blood cells draws down the body's iron stores. The amount and frequency depend on haemoglobin and ferritin monitoring.^[ref] In polycythaemia vera, venesection reduces excessive red blood cell mass and the associated risk of thrombosis.^[ref] These measurable targets are different from the historical idea of “plethora”. In an exhausted person, indiscriminate bloodletting can worsen anaemia and circulatory failure; modern care identifies and treats the cause of the respiratory distress or inflammation instead.

The proposed asymmetry is not generally defensible: animal behaviour and immunity form a reciprocal system, and “more” has no common meaning without an outcome, scale and intervention. Avoiding infection, controlling pathogen growth, tolerating tissue damage and changing social contact are different outcomes. Their causal effect sizes cannot be ranked by counting examples in either direction.

The definition of immunity matters first. In its narrower physiological sense, it concerns cellular and molecular defences. A behavioural immune system describes actions that reduce exposure before those defences are challenged, while social immunity includes protection generated by interactions among group members. If avoidance is defined as part of immunity, a statement that animal behaviour influences immunity becomes partly true by definition. A useful analysis instead specifies whether an action changes exposure, immune activity, pathogen burden or the damage associated with that burden. disease resistance reduces burden; disease tolerance reduces damage at a given burden. Lower illness alone does not distinguish these routes.

One strong route from animal behaviour to defence is the management of exposure. Choice of food, mates and social partners determines contact with infectious material. grooming can remove external parasites, and hygienic behaviour can remove contaminated material from a nest. Conversely, aggregation and some forms of close social contact can increase opportunities for transmission. These effects depend on the pathogen's transmission route: removing one kind of contact need not reduce infection spread by an environmental reservoir. An observed association between sociability and infection can also arise because infection changes sociability, making temporal or experimental evidence essential.

honeybee colonies selected for hygienic behaviour provide a causal evolutionary and experimental example. Selection based on brood-removal assays was combined with exposure to American foulbrood disease, allowing disease outcomes in selected and comparison colonies to be assessed. Removal of infected brood can reduce a colony's infectious reservoir and protect other individuals. ^[ref] The result concerns colony protection, not a demonstration that each worker's cellular immunity is stronger. Selection can change correlated traits as well as brood removal, so a mechanistic follow-up should match colony size, brood age, exposure and background, and manipulate the opportunity for removal. Colonies are independent experimental units; scoring many cells within one colony does not create many independently selected colonies.

Choice of an offspring’s food plant can also change the chemical environment encountered by parasites. Infected monarch butterflies preferentially laid eggs on plants that reduced parasite growth and disease in their offspring. This supported transgenerational medication: neither a simple cure of the mother nor merely a demonstration that a toxic plant can inhibit a parasite. ^[ref] The inference needs both the infection-dependent change in oviposition and the offspring benefit. Tests should include infected and uninfected mothers offered the same plant alternatives, standardized parasite exposure of offspring, and measurements of larval growth and survival as well as parasite burden. Nutritional quality and general plant preference are competing explanations. A cost of the medicinal plant in parasite-free conditions would additionally help explain why every individual does not always choose it.

animal behaviour can influence physiological immunity as well as exposure. Activity, feeding and sleep affect resource acquisition and allocation, while social interactions can alter endocrine and neural signals. These are plausible routes, but a study must measure the relevant immune process and not infer it from survival alone. An immune assay can increase because the animal is more infected, rather than because it has greater protective capacity. randomization, controlled nutrition and repeated observations can help separate these interpretations. Any claim of a trade-off should identify the limiting resource and the competing biological fitness demands; a change in one circulating marker is insufficient.

The reverse direction is equally substantive. Immune challenges can change activity, appetite, social investigation and priorities, producing sickness behaviour. Such changes may conserve resources, reduce exposure to further hazards or alter transmission, but some can instead reflect damaging inflammation or a pathogen's manipulation. Reduced activity after infection is therefore not automatically adaptive host defence. Comparing host biological fitness with and without the response, and separating immune activation from direct pathogen action, is necessary for an evolutionary interpretation.

lipopolysaccharide provides one experimental way to induce an inflammatory response without a replicating pathogen. Work on lactating house mice showed that the resulting effects depended on the activity and environmental context: pup retrieval could be maintained, and cold conditions elicited nest building despite the challenge. ^[ref] This supports a reorganization of competing motivations rather than simple global paralysis. It does not show that all illness-related inactivity is strategic. Controls should match handling and injection, assess motor capacity and temperature, and distinguish loss of appetite from inability to acquire food. Dose and timing also matter: an artificial inflammatory challenge need not reproduce a natural infection's trajectory. Blocking a candidate immune mediator, followed by a rescue intervention, would provide stronger evidence for a specific immune-to-neural pathway than a challenge alone.

Immune influences are not confined to overt sickness. In house mice, work on interferon-gamma linked immune deficiency and altered social preference to changes in neural connectivity and inhibitory physiology. Immune restoration and targeted signalling interventions strengthened a mechanistic interpretation beyond a correlation between illness and social withdrawal. ^[ref] The experiments support a role for immune signalling in the tested neural and behavioural context; they do not establish a universal explanation of sociability or justify extrapolating a house mouse social-preference assay directly to human psychiatric conditions. Sensory ability, general activity, anxiety-related responses, housing and genetic background remain important controls when interpreting such assays.

The two directions can form the same feedback loop. An immune response changes social contact; changed contact alters subsequent exposure and infection; the new infection state changes the immune response again. The medication example similarly links infection-dependent action to offspring defence. Neither direction is an independent input fixed while the other acts. A useful factorial design would manipulate a specific action or opportunity for action and a specific immune challenge, then measure both behavioural and physiological outcomes over time. Baseline health, age, sex and reproductive state should be controlled, and the manipulation of action should avoid introducing injury or stress that independently changes immunity.

A claim that one direction is larger would require comparable interventions and a shared endpoint, such as lifetime reproductive success or pathogen transmission, with uncertainty over an explicitly chosen time horizon. Different answers may follow for individual survival, colony persistence and epidemic spread. The evidence supports bidirectional, context-dependent regulation: actions govern exposure and defence, while immune signals govern behavioural priorities and sometimes normal neural function. A universal ranking conceals precisely the causal distinctions needed to understand this system.

Inflammation of the pericardium; its cause and haemodynamic complications determine management.

Inflammation of the periosteum surrounding a bone. It localizes the affected tissue without determining whether the initiating process is infection, mechanical stress or another cause.

Inflammation of the peritoneum. Infection and chemical irritation are different causes; an intestinal perforation can produce both leakage and a subsequent inflammatory response.

A collection of pus in tissues around a tonsil. The localized cavity differs from uncomplicated diffuse tonsillitis, despite overlapping pain and inflammation.

Inflammation of the pharynx.

Phlebitis is inflammation of a vein. It names an inflammatory process rather than proving thrombosis in every case. A clot-associated process and isolated wall inflammation are related but distinct pathological descriptions.

Inflammation of the pleura.

Pyelonephritis is inflammation, typically infection-related, involving the kidney and its collecting region. It is anatomically distinct from cystitis in the urinary bladder. Both can involve the urinary tract without having identical effects on the affected organ.

A urinary cast containing red blood cells. Its tubular formation supports a renal origin of bleeding and is particularly associated with glomerular inflammation rather than bleeding confined to the lower urinary tract.^[ref]

Cardiac involvement in acute rheumatic fever, particularly valvular inflammation. Pain relief in arthritis does not establish absence of cardiac disease.

A systemic inflammatory disorder that commonly affects synovial anatomical joints. Persistent immune-mediated inflammation can damage cartilage and bone; the disease differs from defining every chronic joint pain as ordinary degeneration.^[ref]

Microbial infection of an anatomical joint. Inflammation can damage articular structures, distinguishing it from noninfectious crystal-associated or autoimmune arthritis.

Spinal cord disease is dysfunction of the spinal cord caused by processes such as injury, compression, inflammation or impaired blood supply. A lesion can affect local circuits as well as pathways carrying signals to and from more distant levels.

A combination of a sensory level, altered reflexes and weakness can therefore be more informative than any one finding. The distribution follows neural organization rather than simply the outline of a single peripheral nerve.

Inflammation of hair follicles, classically in the beard area. Historical usage can include different infectious processes, so the name alone should not be treated as a microbiological identification.

Inflammation of the synovial lining of an anatomical joint. It can cause swelling and contribute to tissue injury through infectious, autoimmune or other mechanisms.

Thyroid disease includes disorders of thyroid structure, secretion or both. Enlargement, inflammation and altered hormone output are different dimensions. A goitre therefore does not by itself establish hypothyroidism or excess secretion.

Inflammation of the tonsils, often associated with infection. Similar throat symptoms can arise from different organisms, so the anatomical label alone does not identify a bacterial cause.