Human Coronaviruses

Slide 1 · Human Coronaviruses

Structure, pathogenesis, pulmonary morphology, and clinicopathologic consequences

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish mild seasonal human coronavirus infections from severe coronavirus pneumonias.

Explain SARS-CoV-2 entry, replication, and evasion of type I interferon defenses.

Relate viral and immune-mediated injury to diffuse alveolar damage, hypoxemia, and thrombosis.

Apply morphology and diagnostic principles to clinical scenarios.

Slide 3 · The clinical spectrum spans two extremes

Usually mild

Endemic human coronaviruses

Upper respiratory tract infection

Cold-like illness

Potentially severe

Highly pathogenic coronaviruses

Lower respiratory tract injury

Pneumonia, diffuse alveolar damage, ARDS

Slide 4 · Coronaviruses are enveloped RNA viruses

Their genome is single-stranded, positive-sense RNA.

A lipid envelope carries proteins needed for host-cell infection.

Human and other vertebrate hosts can be infected.

Schematic virus envelope

Spike projections

Slide 5 · Pathogenicity differs among coronaviruses

Common endemic strains

Mostly restricted to upper airways in the Robbins account

Mild, cold-like presentation

Usually limited tissue injury

SARS, MERS, SARS-CoV-2

Can cause severe lower-respiratory disease

Inflammation amplifies epithelial injury

Severe cases may develop ARDS

Slide 6 · SARS-CoV-2 illustrates severe coronavirus disease

Emergence of SARS-CoV-2

Efficient human transmission

Broad spectrum of COVID-19

COVID-19 emerged in late 2019 and became a global pandemic.

The virus subsequently became endemic in humans.

Slide 7 · SARS and MERS provide clinical contrasts

SARS and MERS

Zoonotic coronaviruses

Severe infections possible

Less efficient sustained human spread account

SARS-CoV-2

Extensive person-to-person transmission

Endemic circulation after pandemic spread

Illness from asymptomatic infection to ARDS

Slide 8 · Transmission occurs through respiratory particles

Respiratory particles emitted

Exposure of susceptible host

Respiratory epithelial infection

Respiratory droplets and aerosols both transmit infection.

Poor ventilation favors accumulation of infectious airborne particles.

Slide 9 · Indoor congregation increases exposure

Prolonged close contact indoors facilitates transmission.

Households and shared residential or care facilities illustrate exposure settings.

Ventilation modifies the environment in which aerosols accumulate.

Shared indoor air

Virus-containing particles

Susceptible respiratory tract

Slide 10 · Asymptomatic infection does not prevent spread

Incubation or no symptoms

Virus present in respiratory tract

Potential transmission to others

Transmission can occur before symptoms and in asymptomatic people.

Symptoms may increase the likelihood of onward spread.

Slide 11 · Incubation precedes the clinical phenotype

Exposure

Incubation

Symptom onset

Early illness

An incubation range of 2–14 days, with median onset around 4–5 days.

Viral load is generally highest early in the infection.

These are source-era observations, not a variant-specific contemporary prediction.

Slide 12 · Host susceptibility strongly modifies disease

Viral exposure

Host susceptibility and immunity

Mild or severe outcome

Older age is a major determinant of severe COVID-19.

Host factors alter whether infection remains controlled or progresses.

Slide 13 · Comorbid disease increases risk

Underlying cardiopulmonary disease and diabetes contribute to severe-disease susceptibility.

Cancer, immunocompromise, and chronic renal or hepatic disease also matter.

Obesity, smoking, and pregnancy are additional risk factors.

Risk reflects a combination of exposure, host biology, and access to care.

Slide 14 · Prior immunity changes the natural history

Immune memory

Faster or stronger protection

Reduced probability of severe disease

Previous infection or vaccination may modify the immune response.

Population immunity and clinical treatment altered disease severity during the pandemic.

Severity estimates from early pandemic periods are not fixed biological constants.

Slide 15 · Coronavirus severity is not a binary property

Virus-associated determinants

Cell entry and tissue tropism

Viral replication and immune evasion

Genetic variation of the virus

Host-associated determinants

Age and comorbidities

Innate antiviral competence

Prior adaptive immunity

Slide 16 · SARS-CoV-2 genome supports direct translation

Positive-sense viral RNA

Host ribosomal translation

Viral proteins and replication machinery

The positive-sense RNA genome is approximately 30 kilobases account.

After entry, host ribosomes translate viral RNA into proteins.

Slide 17 · Three structural components orient the mechanism

Spike protein mediates attachment and membrane fusion.

Membrane protein is located in the viral envelope.

Nucleocapsid protein associates with viral genomic RNA.

Schematic virus envelope

Spike projections

Slide 18 · Spike S1 binds a cellular receptor

Viral spike S1 / RBD

ACE2 on host membrane

Receptor-bound virus

The S1 subunit carries the receptor-binding domain (RBD).

RBD recognition of ACE2 initiates cell-entry events.

Slide 19 · ACE2 helps explain respiratory tropism

ACE2 is expressed on respiratory epithelial cells.

SARS-CoV-2 attachment is enabled where an accessible receptor and entry machinery are present.

Receptor expression alone does not specify clinical severity.

Host epithelial membrane

ACE2

SARS-CoV-2

S1 RBD → ACE2 binding

Slide 20 · TMPRSS2 primes membrane fusion

ACE2 binding by S1

TMPRSS2 cleavage

S2-driven fusion

Viral RNA release

Host-cell protease TMPRSS2 cleaves receptor-engaged spike.

Cleavage activates the S2 fusion machinery.

Viral and host membranes fuse to permit RNA entry.

Slide 21 · Neutralizing antibodies target spike

Antibody–spike binding

Reduced receptor engagement

Entry is prevented

Antibodies recognizing spike, particularly RBD, can block host-cell infection.

This is a mechanism of immune protection, not a direct measure of lung injury.

Slide 22 · Replication begins after viral RNA entry

RNA in cytoplasm

Viral protein synthesis

RNA replication

New viral genomes

Host-cell translation produces replication proteins.

The viral RNA-dependent RNA polymerase copies the genome.

New positive-strand RNAs supply subsequent viral replication.

Slide 23 · Double-stranded RNA is an innate danger signal

Viral replication intermediates

Innate RNA sensing

Type I interferon production

RNA replication generates intermediates recognized as nonself by innate sensors.

Viral RNA structures stimulate type I interferon responses.

These responses inhibit replication in infected and neighboring cells.

Slide 24 · Type I interferon creates an antiviral state

Type I IFN released

Antiviral gene expression

Restriction of viral replication

Interferon signaling activates defenses in infected and nearby cells.

This response opposes viral replication rather than directly neutralizing virions.

Effective early innate defense may contain infection before severe lung injury.

Slide 25 · SARS-CoV-2 evades interferon defenses

Host defense

Detect unusual viral RNA

Produce type I interferon

Induce antiviral signaling

Viral countermeasures

Make RNA resemble host RNA

Shelter replication intermediates

Block signaling leading to interferon

Slide 26 · Viral RNA modification conceals its identity

Viral RNA end processing

Host-like RNA cap

Less innate recognition

The virus modifies and caps its RNA.

These changes make viral RNA more similar to host transcripts.

Reduced recognition by sensors such as RIG-I favors replication.

Slide 27 · Viral endonuclease limits RNA alarm signals

Viral RNA synthesis

Endonuclease action

Fewer detectable RNA intermediates

Viral RNA endonuclease activity minimizes accumulation of negative-strand and double-stranded RNA.

This helps decrease detection of replication intermediates.

Slide 28 · Membrane vesicles shield replicating RNA

Viral proteins recruit endoplasmic reticulum membranes.

Double-membrane compartments contain RNA replication intermediates.

Sequestration limits access of cytoplasmic RNA sensors.

Double-membrane compartment

RNA replication intermediates

Slide 29 · Interferon signaling is also directly inhibited

Viral proteins

Disrupted IFN-production signaling

Delayed antiviral response

Several viral proteins interfere with host proteins involved in interferon-production pathways.

Immune evasion acts at multiple steps rather than through one isolated target.

Slide 30 · Failure of interferon defense predicts vulnerability

Intact type I IFN pathway

Earlier recognition of viral RNA

Antiviral state in affected tissue

Limits spread when effective

Defective type I IFN response

Genetic defects in IFN pathways

Autoantibodies against type I IFN

Increased risk of severe COVID-19

Slide 31 · Virus–host interactions determine lung injury

Epithelial infection

Innate immune response

Local inflammation

Pulmonary dysfunction

Direct viral replication causes epithelial damage.

Host responses amplify inflammation and vascular injury.

The resulting lesion can progress beyond the infected epithelium.

Slide 32 · Upper-airway infection produces mucosal inflammation

Mucosal hyperemia and swelling reflect local inflammation.

Submucosal mononuclear cells include lymphocytes and monocytes.

Excess mucus contributes to congestion and obstruction.

Edematous mucosa + mucus

Schematic airway

Slide 33 · Mucus obstruction explains some complications

Mucosal edema

Viscous secretions

Impaired drainage

Secondary infection

Swelling and viscous secretion can obstruct nasal channels, sinuses, or Eustachian tubes.

Obstruction may facilitate secondary suppurative bacterial infection.

Slide 34 · Waldeyer-ring reactivity is a pediatric feature

Upper airway viral stimulus

Tonsillar immune activation

Reactive lymphoid hyperplasia

Virus-associated tonsillitis can produce lymphoid tissue hyperplasia.

This reflects a local mucosal immune response in children.

Lymphoid hyperplasia is not a coronavirus-specific diagnostic finding.

Slide 35 · Large-airway disease impairs mucociliary function

Epithelial injury

Reduced mucociliary clearance

Accumulation of secretions

Superinfection risk

Viral laryngotracheobronchitis may cause vocal cord edema and mucus hypersecretion.

Compromised bronchociliary clearance favors bacterial superinfection.

Slide 36 · Bronchiolar plugging has mechanical effects

Cell debris, fibrin, mucus, and inflammatory exudate can obstruct small airways.

Segmental or focal atelectasis can follow distal airway blockage.

Severe prolonged damage may heal by organization and fibrosis.

Intraluminal debris / exudate

Schematic airway

Slide 37 · Persistent bronchiolitis can leave structural damage

Bronchiolar exudate

Organization and fibrosis

Fixed luminal narrowing

Prolonged exudate organization may narrow or obliterate airway lumina.

This pathway can lead to obliterative bronchiolitis and permanent impairment.

It is a severe viral airway outcome, not inevitable in routine COVID-19.

Slide 38 · Viral pneumonia need not be spatially uniform

Patchy involvement

Discrete affected parenchymal regions

Adjacent relatively aerated lung

Heterogeneous ventilation

Extensive involvement

Lobar or bilateral disease possible

Red-blue congested lung tissue

Greater loss of functioning gas exchange

Slide 39 · Interstitial inflammation is a central pattern

Alveolar septa become widened and edematous.

Mononuclear inflammatory cells infiltrate alveolar walls.

The process is centered on the interstitium, not simply the alveolar lumen.

Widened, inflamed alveolar septum

Schematic — not histopathology

Slide 40 · The interstitial infiltrate is usually mononuclear

Typical viral interstitial process

Lymphocytes and macrophages

Occasional plasma cells

Septal edema and widening

Severe or modified pattern

Neutrophils may also be present

Superimposed bacteria change exudate composition

Histology alone may not identify the virus

Slide 41 · Alveolar exudate varies across cases

Some viral pneumonias have minimal intraalveolar material.

Others contain protein-rich fluid and inflammatory cells.

Increasing exudation compromises aeration and diffusion.

Proteinaceous intraalveolar exudate

Schematic — not histopathology

Slide 42 · Severe COVID-19 may produce congested, edematous lungs

In fatal cases, gross lungs are congested and edematous.

Vascular congestion accompanies alveolar and septal fluid accumulation.

These findings are not specific to one viral pathogen.

Alveolar edema and capillary congestion

Schematic — not histopathology

Slide 43 · Diffuse alveolar damage defines the ARDS lesion

Alveolar–capillary injury

Barrier leak + epithelial necrosis

Hyaline membranes / DAD

Severe epithelial and endothelial injury culminates in diffuse alveolar damage (DAD).

Hyaline membranes, edema, and epithelial injury characterize the exudative phase.

DAD represents an injury pattern, not a pathogen-specific signature.

Slide 44 · Hyaline membranes form from leaked proteins and debris

Capillary leak delivers fibrin-rich plasma proteins into alveolar spaces.

Necrotic epithelial debris mixes with the exudate.

Condensed material lines alveolar walls as eosinophilic hyaline membranes.

Hyaline membrane lining

Schematic — not histopathology

Slide 45 · Type II pneumocyte hyperplasia signals epithelial repair

Type I pneumocyte injury

Type II cell proliferation

Attempted epithelial restitution

Severe COVID-19 may show hyperplasia of type II pneumocytes.

Surviving type II cells proliferate during alveolar epithelial restoration.

This accompanies evolving or organizing diffuse alveolar damage.

Slide 46 · Pulmonary microthrombi link inflammation to perfusion

Thrombi occur in pulmonary arterioles and alveolar capillaries in severe fatal COVID-19.

Vascular thrombosis adds perfusion abnormalities to impaired alveolar ventilation.

The coexistence of DAD and microthrombi is diagnostically informative but not exclusive.

Capillary thrombi

Schematic — not histopathology

Slide 47 · Acute lung injury can enter an organizing phase

Acute exudative DAD

Type II cell response

Organization / tissue repair

The acute phase shows edema, epithelial necrosis, fibrin, and hyaline membranes.

The organizing phase includes type II pneumocyte hyperplasia and granulation tissue.

The trajectory depends on persistence or resolution of injury.

Slide 48 · Recovery and fibrosis are alternative trajectories

Recovery

Trigger and inflammation abate

Macrophages clear debris

Alveolar lining reconstituted

Fibroproliferative outcome

Persistent injury and organizing exudate

Fibroblast activation and collagen deposition

Possible chronic functional impairment

Slide 49 · Ventilation–perfusion mismatch causes hypoxemia

Interstitial/alveolar injury

Poor aeration with ongoing perfusion

Hypoxemia

Edema and alveolar exudate reduce ventilation of perfused regions.

Patchy parenchymal injury creates regional V/Q inequality.

Gas exchange failure can be prominent despite limited focal signs.

Slide 50 · Symptoms can exceed the physical examination

Reported symptoms

Fever and myalgia

Headache and fatigue

Dyspnea or cough; Cough may be absent

Pathologic explanation

Interstitial edema and inflammation

V/Q mismatch

Impaired oxygenation despite scant localizing signs

Slide 51 · Clinical severity ranges from silent to critical

Asymptomatic or mild

Moderate respiratory illness

Hypoxemic pneumonia

ARDS / systemic disease

Asymptomatic infection can occur.

Most recognized cases present with an acute respiratory/systemic illness.

Severe cases progress to hypoxemia and respiratory failure.

Slide 52 · Severe disease includes hyperinflammation

Dysregulated host response

Cytokine amplification

Pulmonary and systemic injury

Severe COVID-19 may show elevated proinflammatory cytokines, including TNF and IL-6.

These signals amplify inflammatory cell activation and tissue injury.

Inflammation is one component of disease, not the sole explanation.

Slide 53 · Lymphopenia reflects immune dysregulation

Clinical laboratory pattern

Reduced circulating lymphocytes

Altered myeloid cell populations

Reduced T- and NK-cell numbers in severe disease

Interpretive caution

Supports systemic immune disturbance

Not a standalone diagnostic test

Must be interpreted with infection confirmation and clinical context

Slide 54 · COVID-associated coagulopathy promotes thrombosis

Endothelial injury

Tissue factor / clotting activation

Pulmonary microthrombi

Inflammation or endothelial damage can expose tissue factor.

Coagulation activation favors intravascular thrombus formation.

Vascular injury and lung injury can reinforce each other.

Slide 55 · Reduced fibrinolysis may intensify thrombosis

PAI-1 elevation

Reduced plasmin activation

Impaired fibrinolysis

Thrombus persistence

PAI-1 can inhibit tissue plasminogen activator and urokinase.

Less plasmin generation impairs fibrin degradation.

The combined effect can shift the balance toward thrombosis.

Slide 56 · MIS-C is a delayed pediatric inflammatory syndrome

Prior SARS-CoV-2 infection

Delayed inflammatory response

Multisystem dysfunction

Onset commonly 2–6 weeks after acute COVID-19.

Gastrointestinal, mucocutaneous, neurologic, and cardiac abnormalities may coexist.

Shock can occur, but most affected children recover.

Slide 57 · Adult systemic inflammation has diverse patterns

Some severe adult cases show an HLH-like syndrome with cytopenias and acute-phase reactant elevation.

Vasculitis may involve skin, CNS, lungs, or gastrointestinal tract.

Myositis and arthritis may accompany systemic inflammation.

These complications should not be inferred from pulmonary histology alone.

Slide 58 · Long COVID extends beyond acute viral pneumonia

Physical symptoms described

Fatigue and weakness

Dyspnea and chest pain

Cough

Neuropsychological symptoms

Impaired memory or concentration

Anxiety and depression

Posttraumatic stress manifestations

Slide 59 · Long COVID trajectories are heterogeneous

Acute COVID-19

Postacute symptoms

Recovery, relapse, or persistence

Symptoms may persist or relapse after acute infection.

Definitions vary across studies, complicating prevalence estimates.

Some recover while others have relapsing-remitting or progressive symptoms.

Slide 60 · Bacterial superinfection changes morphology

Predominantly viral injury

Interstitial mononuclear infiltrate

Variable proteinaceous alveolar exudate

Possible diffuse alveolar damage

Superimposed bacterial infection

Suppurative bronchitis / bronchiolitis

More neutrophil-rich exudation

Bacterial pneumonia may become dominant

Slide 61 · Antigen tests and PCR detect different analytes

Antigen assay

Antibody-based detection of nucleocapsid protein

Usually performed on upper-airway swabs

Detects viral protein

Molecular assay

PCR-based detection of viral RNA

Targets nucleic acid rather than protein

Method performance depends on specimen and timing

Slide 62 · Genetic changes continually generate variants

Viral RNA variation

Altered viral proteins or biology

Changed population-level behavior

Mutations, insertions, deletions, and recombination change viral sequences.

Variant phenotype depends on effects on transmissibility, pathogenicity, and immune recognition.

Slide 63 · Spike changes can modify immune escape

Spike sequence change

Reduced antibody recognition

Potential reinfection advantage

Changes in spike may impair recognition by existing neutralizing antibodies.

The textbook uses the emergence of Omicron as an example of immune escape.

A named historical variant should not be mistaken for the current dominant lineage.

Slide 64 · Clinical application: Mild respiratory infection

Clinical problem

A patient has coryza, sore throat, and mild cough without hypoxemia.

What epithelial changes best explain congestion and impaired drainage?

Why should a histology pattern not be labeled coronavirus-specific?

Teaching interpretation

Mucosal hyperemia/edema, submucosal mononuclear inflammation, and mucus hypersecretion; Similar findings occur with many respiratory viruses.

Slide 65 · Clinical application: Rapidly progressive hypoxemia

Clinical problem

An older patient with comorbid illness develops dyspnea and diffuse opacities after confirmed SARS-CoV-2 infection.

Which pattern of lung injury should be considered?

How do hyaline membranes and microthrombi affect gas exchange?

Teaching interpretation

Diffuse alveolar damage with alveolar edema/hyaline membranes, often with pulmonary microthrombi; Both ventilation and perfusion become disturbed.

Slide 66 · Morphology exercise: Diagnose an injury pattern

Clinical problem

Lung sections show septal edema, intraalveolar fibrin-rich exudate, hyaline membranes, and reactive type II pneumocytes.

Name the morphologic pattern; Then identify what this pattern cannot establish by itself.

Which additional data would support a SARS-CoV-2 etiology?

Teaching interpretation

Diffuse alveolar damage. It is nonspecific as to etiology; Clinical context and SARS-CoV-2 antigen/PCR results support causation.

Slide 67 · Integrating mechanisms with diagnostic findings

Early / less severe infection

Virus attaches and replicates

Upper airway symptoms predominate

Mucosal edema and mononuclear reaction

Severe pulmonary involvement

Immune dysregulation and vascular injury

Hypoxemia and possible ARDS

DAD, hyaline membranes, microthrombi

Slide 68 · Core conclusions

Human coronaviruses span mild upper-airway infections and severe lower-respiratory disease.

ACE2–spike binding and TMPRSS2-dependent fusion initiate SARS-CoV-2 entry.

Type I interferon defenses oppose replication; Viral proteins interfere with recognition and signaling.

Severe lung disease is mediated by epithelial injury, inflammation, diffuse alveolar damage, and vascular thrombosis.

Slide 69 · References and source boundaries

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. 2026. Sections: Human Coronaviruses and viral pneumonia.

Robbins, Cotran & Kumar. Pathologic Basis of Disease. Section: COVID-19 (Infectious Diseases).

Robbins, Cotran & Kumar. Pathologic Basis of Disease. Section: Acute Lung Injury and ARDS (The Lung).

Schematic visuals are original editable teaching diagrams; no clinical images or histology specimens are presented.