Infection Risk Factors: Host, Pathogen & Environmental Vulnerabilities

Susceptibility to infection depends on host, pathogen, and environmental factors. Key risk categories:

1. Age Extremes

• Infants/young children: Immature immunity, incomplete vaccination, exploratory behavior.
• Older adults (≥65): Immunosenescence (↓ T-cell function, ↓ vaccine response), comorbidities, polypharmacy, institutionalization.

2. Chronic Diseases

• Diabetes mellitus: Hyperglycemia impairs neutrophil chemotaxis/phagocytosis; vascular/neuropathic complications.
• HIV/AIDS: CD4+ depletion → opportunistic infections.
• Cancer/hematologic malignancies: Disease + chemo/radiation → neutropenia, barrier breakdown.
• Chronic kidney/liver/lung disease: Uremic toxins, cirrhosis immune dysfunction, impaired clearance.
• Autoimmune/rheumatic diseases: Intrinsic dysregulation + immunosuppressants.

3. Immunosuppression (Iatrogenic or Disease-Related)

• Corticosteroids (≥20 mg prednisone/day ≥2 weeks)
• Biologics (anti-TNF, anti-IL-6, anti-CD20, JAK inhibitors)
• Transplant immunosuppressants (tacrolimus, mycophenolate)
• Chemotherapy (neutropenia nadir 7–14 days post-cycle)

4. Barrier Defects

• Skin: burns, wounds, ulcers, dermatitis, lines/tubes.
• Mucosal: intubation, catheterization, surgery, mucositis.

5. Lifestyle & Social Factors

• Malnutrition (protein-energy, micronutrients: Zn, Vit A, D)
• Smoking (impaired ciliary clearance, macrophage function)
• Alcohol excess (immune suppression, aspiration risk)
• Crowding, homelessness, poor sanitation
• Travel to endemic areas

6. Genetic/Primary Immunodeficiencies

• CGD, CVID, SCID, complement deficiencies, etc.

Clinical Application

Risk stratification guides vaccination (inactivated vs. live), prophylaxis (TMP-SMX for PJP, antivirals), antimicrobial stewardship, and infection control precautions.