Chapter 24: Microbial Diseases of the Respiratory System

The chapter begins with a review of the respiratory system’s defenses—mucociliary escalator, alveolar macrophages, and secretory IgA—which normally prevent pathogen colonization. However, when these barriers are breached, various bacteria, viruses, and fungi can cause inflammation and disease. The upper respiratory system is vulnerable to infections like pharyngitis, laryngitis, and tonsillitis, many caused by Streptococcus pyogenes. Of particular concern is streptococcal pharyngitis (strep throat) and its complications—scarlet fever and rheumatic fever, an autoimmune reaction that can damage heart valves. Diphtheria, caused by Corynebacterium diphtheriae, produces a powerful exotoxin and a thick gray pseudomembrane in the throat; vaccination via DTaP is critical for prevention. Otitis media (middle ear infection) is commonly caused by S. pneumoniae, H. influenzae, or M. catarrhalis and frequently affects children. For the lower respiratory tract, the chapter covers pertussis (whooping cough), caused by Bordetella pertussis, and its progression through catarrhal, paroxysmal, and convalescent stages. Vaccination with the DTaP series has significantly reduced its incidence. Tuberculosis, caused by Mycobacterium tuberculosis, is covered in depth—its pathogenesis, formation of tubercles, latent vs active disease, diagnostic tests (TST, IGRA), and long-course multi-drug treatment regimens. Bacterial pneumonias include those caused by Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae (walking pneumonia), Legionella pneumophila (Legionnaires’ disease), and Chlamydophila species. Each pathogen’s features, transmission, and treatment are compared. The viral infections reviewed include the common cold, mainly due to rhinoviruses and coronaviruses, respiratory syncytial virus (RSV), influenza, and SARS/MERS. The structure, antigenic shifts and drifts in influenza viruses, and vaccine development are highlighted. Fungal diseases of the respiratory tract are also covered, including histoplasmosis, coccidioidomycosis, blastomycosis, and pneumocystis pneumonia (PCP)—a common opportunistic infection in AIDS patients. The chapter concludes with notes on emerging pathogens and the importance of respiratory hygiene, vaccination, and surveillance.