A public health officer is comparing the epidemiologic patterns of tuberculosis (TB) and measles in a rural Indian district. Which feature best distinguishes the web of causation of TB from that of measles?

Explanation

## Distinguishing Features of TB vs Measles in the Epidemiologic Triad ### Web of Causation: TB **Key Point:** TB exemplifies a **complex, multifactorial web of causation** where the agent (Mycobacterium tuberculosis) is necessary but not sufficient. 1. **Agent:** M. tuberculosis (obligate pathogen) 2. **Host factors:** Malnutrition, immunosuppression (HIV/AIDS), diabetes, chronic lung disease, alcoholism, smoking 3. **Environmental factors:** Overcrowding, poor ventilation, poverty, inadequate healthcare access 4. **Temporal pattern:** Chronic disease with slow progression; latent TB can reactivate years later ### Web of Causation: Measles **Key Point:** Measles represents a **relatively simple, agent-dominant model** where the measles virus is both necessary and sufficient in most cases. 1. **Agent:** Measles virus (highly contagious, R₀ = 12–18) 2. **Host factors:** Primarily vaccination status and age; most infected individuals develop disease regardless of socioeconomic status 3. **Environmental factors:** Minimal role; transmission occurs in any setting with susceptible contacts 4. **Temporal pattern:** Acute disease with rapid onset; immunity (natural or vaccine-induced) is protective ### Comparison Table | Feature | TB | Measles | |---------|----|---------| | **Causation model** | Multifactorial web | Agent-dependent | | **Role of poverty/malnutrition** | Critical cofactor | Minimal | | **Immunization impact** | Partial (BCG ~70% efficacy) | Near-complete (>95% with 2 doses) | | **Disease progression** | Chronic, slow | Acute, rapid | | **Latency** | Yes (years) | No | | **Reactivation** | Common | Rare/absent | **High-Yield:** TB requires the **convergence of agent + host vulnerability + environmental stress**, whereas measles is primarily **agent-driven** — the virus alone causes disease in susceptible individuals regardless of socioeconomic context. **Clinical Pearl:** This distinction explains why TB control requires multisectoral approaches (nutrition, housing, TB drugs, HIV management), while measles control is primarily vaccination-driven. [cite:Park 26e Ch 5]