How Viruses Harm the Human Body: Mechanisms of Pathogenesis

Viruses are obligate intracellular parasites. Pathogenesis involves:

1. Entry & replication: Bind receptors → enter → hijack host machinery → replicate → assemble → exit (lysis or budding).
2. Direct cytopathic effects: Cell lysis, apoptosis, necrosis, syncytia formation, inhibition of host macromolecular synthesis (e.g., influenza NS1 blocks host mRNA processing).
3. Immune-mediated damage: Cytotoxic T cells kill infected cells; antibody-dependent enhancement (ADE); immune complexes (vasculitis); cytokine storm (excessive IL-6, TNF-α, IFN-γ → ARDS, multi-organ failure — seen in severe COVID-19, influenza).
4. Latency & reactivation: Herpesviruses (HSV, VZV, EBV, CMV) establish latency; reactivate under immunosuppression → disease.
5. Oncogenesis: HPV (E6/E7 degrade p53/Rb), HBV/HCV (chronic inflammation → cirrhosis → HCC), EBV (Burkitt lymphoma, NPC), HTLV-1 (ATLL), KSHV (Kaposi sarcoma).
6. Secondary infections: Viral damage to barriers/immunity → bacterial superinfection (post-influenza pneumonia).

Severity depends on virus virulence, inoculum, host genetics, age, immunity, comorbidities. Vaccines and antivirals target specific steps.