Host Susceptibility to Infectious Diseases: Eight Key Factors

Susceptibility results from interplay between pathogen virulence and host defenses. Major determinants:

1. Immune Competence

Infants (immature immunity), elderly (immunosenescence), immunocompromised (HIV, chemotherapy, biologics, transplant) have reduced pathogen clearance.

2. Lack of Specific Immunity

Naïve immune system (no prior exposure/vaccination) lacks memory T/B cells and antibodies for rapid response.

3. High Inoculum Exposure

Overwhelming dose (e.g., needlestick, aerosol-generating procedure) can breach even robust defenses.

4. Environmental Conditions

Poor sanitation, crowding, contaminated water/food, vector exposure amplify transmission and dose.

5. Lifestyle Factors

Smoking (impaired mucociliary clearance), alcohol (immune suppression), malnutrition (protein/ micronutrient deficiency), sleep deprivation (reduced cytokine production).

6. Genetic Variants

HLA alleles, CCR5-Δ32, FUT2 (norovirus), sickle trait (malaria), TLR polymorphisms alter infection risk/severity.

7. Iatrogenic Immunosuppression

Corticosteroids, chemotherapy, TNF-α inhibitors, calcineurin inhibitors deliberately or inadvertently impair immunity.

8. Novel Pathogen Exposure

Travel to endemic zones introduces pathogens without pre-existing immunity (e.g., dengue, malaria, novel influenza).

Risk Mitigation

Vaccination, hygiene, vector control, antimicrobial stewardship, and comorbidity optimization reduce susceptibility.