Microbial Diseases of the Respiratory System: Bacterial, Viral, and Fungal Infections

Bacterial Diseases of the Upper Respiratory System, Sinuses, and Ears

Streptococcal Respiratory Diseases

Streptococcal infections are common causes of pharyngitis and related complications in the upper respiratory tract.

• Signs & Symptoms: Inflammation of pharynx, sore throat, difficulty swallowing, purulence on tonsils, fever, malaise, headache.

• Complications: Can lead to scarlet fever, acute glomerulonephritis, rheumatic fever, and bronchitis if spread to lower respiratory tract.

• Pathogen: Streptococcus pyogenes (Group A Streptococcus).

• Virulence Factors:

  • M proteins: Inhibit complement component.

  • Hyaluronic acid capsule: Camouflages bacterium.

  • Streptokinases: Break down blood clots.

  • C5a peptidase: Breaks down complement protein.

  • Pyrogenic toxins: Stimulate leukocytes, cause fever, rash, shock.

  • Streptolysins: Lyse erythrocytes, leukocytes, and platelets.

• Pathogenesis: Disease occurs when normal microbiota are missing, large inoculum is present before antibodies form, or adaptive immunity is impaired.

• Transmission: Respiratory droplets; most common in winter and spring.

• Diagnosis: Serological testing.

• Treatment: Antibiotics.

• Prevention: No vaccine available.

Diphtheria

Diphtheria is a potentially deadly childhood disease characterized by the formation of a pseudomembrane in the throat.

• Signs & Symptoms: Sore throat, localized pain, pharyngitis, fever, pseudomembrane obstructing airways.

• Pathogen: Corynebacterium diphtheriae.

• Virulence Factors: Diphtheria toxin inhibits protein synthesis, causing cell death; V-shaped arrangement from snapping division.

• Pathogenesis: Severity depends on host immune status; can be asymptomatic or severe in immunocompromised patients.

• Transmission: Respiratory droplets and skin contact; most common in nonimmune children.

• Diagnosis: Presence of pseudomembrane.

• Treatment: Antitoxins and antibiotics.

• Prevention: Immunization.

Rhinosinusitis & Otitis Media

These are infections of the nasal passages and middle ear, often caused by bacteria spreading from the pharynx.

• Rhinosinusitis: Malaise, headache, inflamed nasal passage.

• Otitis Media: Severe ear pain, possible eardrum rupture, hearing loss.

• Pathogens: Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pyogenes.

• Pathogenesis: Infection via auditory tube; inflammation causes symptoms.

• Transmission: Bacteria spread from pharynx to sinuses.

• Epidemiology: Rhinosinusitis common in adults; otitis media in children.

• Diagnosis: Based on symptoms.

• Treatment: Antibiotics; cough suppressants for rhinosinusitis.

• Prevention: None.

Viral Diseases of the Upper Respiratory System

Common Cold

The common cold is a mild viral infection affecting the upper respiratory tract.

• Signs & Symptoms: Sneezing, runny nose, congestion, sore throat, malaise, cough; fever only with secondary infection.

• Pathogens: Enteroviruses (rhinoviruses) most common; many viruses can cause the common cold.

• Pathogenesis: Virus attaches to nasal mucosa, replicates, kills cells, triggers inflammation.

• Transmission: Coughing, sneezing, fomites.

• Epidemiology: Occurs in about 50% of people.

• Diagnosis: Based on signs and symptoms.

• Treatment: Symptomatic relief.

• Prevention: Hand washing; no effective vaccine due to many strains.

Bacterial Diseases of the Lower Respiratory System

Pneumococcal Pneumonia

Pneumococcal pneumonia is the most common type of bacterial pneumonia.

• Signs & Symptoms: Fever, chills, cough, congestion, chest pain, rapid breathing, rust-colored sputum if blood enters lungs.

• Pathogen: Streptococcus pneumoniae.

• Virulence Factors: Capsule, phosphorylcholine (allows uptake by lung cells), pneumolysin (cytotoxin).

• Pathogenesis: Bacteria multiply in alveoli, decrease gas exchange, kill lung cells.

• Transmission: Inhalation of bacteria.

• Epidemiology: Most common in immunocompromised individuals.

• Diagnosis: Diplococci in sputum smears.

• Treatment: Antibiotics (resistance is increasing).

• Prevention: Vaccination.

Primary Atypical (Mycoplasmal) Pneumonia

Also known as "walking pneumonia," this disease is mild but can last for weeks.

• Signs & Symptoms: Fever, malaise, sore throat, excessive sweating.

• Pathogen: Mycoplasma pneumoniae.

• Virulence Factors: Adhesive protein, capsule; stops ciliary movement, kills epithelial cells.

• Pathogenesis: Disrupts mucus movement, allows colonization by other bacteria.

• Transmission: Nasal secretions.

• Epidemiology: Common in high school and college students.

• Diagnosis: Difficult due to small size of mycoplasma.

• Treatment: Antibiotics.

• Prevention: Difficult; asymptomatic carriers.

Klebsiella Pneumonia

Klebsiella pneumonia is more severe and has a higher mortality rate than other types.

• Signs & Symptoms: Fever, cough, congestion, chest pain, bloody sputum, chills.

• Pathogen: Klebsiella pneumoniae (Gram-negative, opportunistic).

• Virulence Factors: Capsule, endotoxin release upon lysis.

• Pathogenesis: Kills alveolar cells, invades blood.

• Transmission: Not specified; occurs mostly in immunocompromised patients.

• Diagnosis: Identification in sputum samples.

• Treatment: Antimicrobials.

• Prevention: Aseptic technique.

Other Bacterial Pneumonias

• Haemophilus influenzae & Staphylococcus aureus: Similar to pneumococcal pneumonia.

• Yersinia pestis: Causes pneumonic plague.

• Chlamydia psittaci: Causes psittacosis (parrot fever).

• Chlamydia pneumoniae: Causes pneumonia, bronchitis, rhinosinusitis, mild infections.

Legionnaires Disease

Legionnaires disease is a severe form of pneumonia caused by Legionella pneumophila.

• Signs & Symptoms: Fever, chills, cough, congestion, chest pain, rapid breathing, rapid pulmonary decline, rust-colored sputum if blood enters lungs.

• Pathogen: Legionella pneumophila.

• Virulence Factors: Intracellular growth, tissue damage, inflammation.

• Transmission: Survives in water sources, resistant to heat and chlorine.

• Epidemiology: Elderly, smokers, immunocompromised.

• Diagnosis: Fluorescent antibody staining, serology.

• Treatment: Antibiotics.

• Prevention: Reduce bacteria in water.

Tuberculosis

Tuberculosis is a chronic, potentially fatal disease caused by Mycobacterium tuberculosis.

• Signs & Symptoms: Minor cough, mild fever; later difficulty breathing, fatigue, malaise, weight loss, chest pain, coughing blood.

• Pathogen: Mycobacterium tuberculosis (mycolic acid gives unique features).

• Virulence Factors: Slow growth, protection from phagocytosis, intracellular growth, resistance to antimicrobials.

• Pathogenesis: Forms tubercles in lungs; can reactivate or disseminate to other organs.

• Transmission: Inhalation of droplets.

• Epidemiology: 1/3 of world population infected.

• Diagnosis: Tuberculin skin test, chest X-ray.

• Treatment: Combination drug therapy.

• Prevention: BCG vaccine in endemic areas.

Pertussis (Whooping Cough)

Pertussis is a highly contagious disease characterized by severe coughing fits.

• Signs & Symptoms: Cold-like symptoms, then severe cough.

• Pathogen: Bordetella pertussis.

• Virulence Factors: Adhesions, toxins, interferes with ciliary movement, triggers excess mucus.

• Pathogenesis: Disease progresses through incubation, catarrhal, paroxysmal, and convalescent phases.

• Transmission: Respiratory droplets.

• Epidemiology: Immunocompromised children; highly contagious.

• Diagnosis: Symptoms.

• Treatment: Supportive care.

• Prevention: DTaP vaccine.

Inhalation Anthrax

Inhalation anthrax is a rare but deadly disease caused by inhaling endospores.

• Signs & Symptoms: Initial cold-like symptoms, then severe coughing, nausea, vomiting, fainting, confusion, lethargy, shock; death within days if untreated.

• Pathogen: Bacillus anthracis (Gram-positive, endospore-forming).

• Virulence Factors: Anthrax toxin (cytotoxin), endospores germinate and secrete toxin.

• Transmission: Inhalation of endospores from animal hides/wool.

• Epidemiology: People working with animal hides/wool.

• Diagnosis: Identification of Gram-positive bacteria in sputum.

• Treatment: Obiltocaximab; antimicrobials may be effective but can cause lung damage.

• Prevention: Vaccines for select populations.

Viral Diseases of the Lower Respiratory System

Influenza (Flu)

Influenza is a highly contagious viral disease with sudden onset of fever and respiratory symptoms.

• Signs & Symptoms: Pharyngitis, congestion, dry cough, malaise, headache, myalgia; sudden fever distinguishes flu from common cold.

• Pathogens: Influenza Type A & B viruses.

• Virulence Factors: Lipid envelope, glycoprotein spikes (hemagglutinin HA, neuraminidase NA).

• Genetic Variation:

  • Antigenic Drift: Accumulation of mutations in HA and NA genes, creating new strains.

  • Antigenic Shift: Reassortment of genes from different Influenza A viruses infecting the same cell.

• Pathogenesis: Virus replicates in lung epithelial cells, damages tissue, increases susceptibility to bacterial infections.

• Diagnosis: Signs/symptoms, community outbreak.

• Treatment: Supportive care.

• Prevention: Multivalent vaccine, personal hygiene.

Coronavirus Respiratory Syndrome

Coronaviruses cause mild to severe respiratory diseases, including SARS and MERS.

• Signs & Symptoms: High fever, labored breathing, malaise, body aches, pneumonia with dry cough.

• Pathogen: Coronavirus.

• Pathogenesis: Destroys lung cells, can spread to heart and kidneys.

• Transmission: Respiratory droplets.

• Epidemiology: SARS mortality 10%, MERS mortality 55%.

• Diagnosis: Signs/symptoms.

• Treatment: Supportive care.

• Prevention: Vaccine development ongoing.

Respiratory Syncytial Virus Infection (RSV)

RSV is the most common respiratory disease in newborns and young children.

• Signs & Symptoms: Fever, rhinorrhea, cyanosis, coughing, wheezing; leading cause of bronchitis and pneumonia in children.

• Pathogen: Respiratory Syncytial Virus.

• Virulence Factors: Causes syncytia (giant cells), plugs bronchioles with mucus and dead cells.

• Pathogenesis: Immune response further damages lungs.

• Transmission: Close contact, fomites.

• Epidemiology: Common in immunocompromised, elderly, babies.

• Diagnosis: Immunoassay.

• Treatment: Supportive care.

• Prevention: Aseptic technique by healthcare workers.

Hantavirus Pulmonary Syndrome (HPS)

HPS is a severe viral disease transmitted from rodents, causing pneumonia and shock.

• Signs & Symptoms: Fever, fatigue, muscle aches, then coughing, shock, difficulty breathing as lungs fill with fluid.

• Pathogen: Hantavirus (infects mice).

• Pathogenesis: Virus infects capillary wall cells, inflammation causes capillary leakage and low blood pressure.

• Transmission: Inhalation of dried mouse urine, feces, saliva.

• Epidemiology: Most common west of the Mississippi; 50% mortality.

• Diagnosis: Disease manifestations, serologic/PCR assays.

• Treatment: Supportive care.

• Prevention: Avoid exposure.

Other Viral Respiratory Diseases

• Metapneumovirus (MPV): Second most common cause of viral respiratory disease; antibodies present in all children by age 5.

• Parainfluenza Virus: Three strains cause croup and viral pneumonia; most common in young children.

• Treatment: None.

• Prevention: Frequent hand washing.

Mycoses of the Lower Respiratory System

Coccidioidomycosis (Valley Fever)

Coccidioidomycosis is a fungal disease resembling TB or pneumonia, with potential for severe systemic infection.

• Signs & Symptoms: TB/pneumonia-like symptoms, large lesions, CNS involvement (meningitis), headache, nausea, emotional disturbance, inflamed bone/joint tissues.

• Pathogen: Coccidioides immitis.

• Pathogenesis: Asexual spores cause pulmonary infection, spread via spherules.

• Transmission: Inhalation of spores from disrupted soil.

• Epidemiology: Southwestern US, Northern Mexico.

• Diagnosis: Spherules in samples, antigen skin test.

• Treatment: IV antifungal drugs.

• Prevention: Masks in endemic areas, avoid soil exposure.

Blastomycosis

Blastomycosis is a soil-dwelling fungal infection that can cause chronic or fatal disease.

• Signs & Symptoms: Flulike symptoms, skin lesions, necrosis, cavity formation in bones/tissues.

• Pathogen: Blastomyces dermatitis (dimorphic fungus).

• Pathogenesis: Lesions may resolve in healthy people; disease can become chronic/fatal.

• Transmission: Dust carrying fungal spores.

• Epidemiology: Immunocompromised individuals.

• Diagnosis: Dimorphism in lab cultures, microscopic examination.

• Treatment: Antifungal drugs for 10+ weeks.

• Prevention: Avoid infected areas.

Histoplasmosis

Histoplasmosis is a dimorphic fungal infection, often asymptomatic but can cause severe disease in immunocompromised individuals.

• Signs & Symptoms: Severe coughing, blood-tinged sputum, skin lesions; in AIDS patients, enlarged liver/spleen, eye inflammation.

• Pathogen: Histoplasma capsulatum (dimorphic ascomycete).

• Pathogenesis: Intracellular parasite attacks alveolar macrophages; infected macrophages spread fungus via blood/lymph.

• Transmission: Droppings from bats/birds in moist soil.

• Epidemiology: People working with bats/birds.

• Diagnosis: Fungal cells and macrophages in tissue samples.

• Treatment: None or antifungal drugs.

• Prevention: Avoid infected areas, protect self.

Pneumocystis Pneumonia (PCP)

PCP is a fungal infection, usually asymptomatic in healthy individuals but severe in immunocompromised patients.

• Signs & Symptoms: Difficulty breathing, mild anemia, hypoxia, fever.

• Pathogen: Pneumocystis jirovecii (formerly P. carinii).

• Pathogenesis: Normal respiratory microbiota; body clears fungus in healthy individuals.

• Transmission: Inhalation of droplet nuclei containing fungus.

• Epidemiology: Immunocompromised individuals.

• Diagnosis: Chest X-rays, fungus in tissues (cysts) or fluids.

• Treatment: Antifungals.

• Prevention: Impossible to prevent.

Summary Table: Respiratory Diseases by Etiology

Disease	Etiological Agent	Key Symptoms	Diagnosis	Treatment	Prevention
Strep Throat	Streptococcus pyogenes	Sore throat, fever, purulence	Serological testing	Antibiotics	None
Diphtheria	Corynebacterium diphtheriae	Sore throat, pseudomembrane	Pseudomembrane presence	Antitoxins, antibiotics	Immunization
Pneumococcal Pneumonia	Streptococcus pneumoniae	Fever, cough, chest pain	Sputum smear	Antibiotics	Vaccination
Influenza	Influenza A & B	Sudden fever, malaise	Symptoms, outbreak	Supportive care	Multivalent vaccine
RSV Infection	Respiratory Syncytial Virus	Cough, wheezing, cyanosis	Immunoassay	Supportive care	Aseptic technique
Coccidioidomycosis	Coccidioides immitis	TB-like, lesions, meningitis	Spherules, skin test	IV antifungals	Masks, avoid soil
PCP	Pneumocystis jirovecii	Difficulty breathing, hypoxia	Chest X-ray, cysts	Antifungals	None