viral infections
Summary
Viral infections occur when obligate intracellular pathogens (viruses) invade host cells, hijack cellular machinery for replication, and trigger immune responses. Clinical presentations range from asymptomatic to life-threatening depending on viral tropism, replication strategy, and host immune status. Key USMLE topics include viral structure/classification, replication cycles, and pathogen-specific clinical syndromes.
Detail
Viruses are classified by genome type (DNA vs RNA), strandedness, polarity (positive vs negative sense RNA), presence of envelope, and capsid symmetry. This classification predicts replication strategy, site of replication (nucleus vs cytoplasm), and susceptibility to antivirals/disinfectants.
Pathophysiology: Viruses attach via specific receptors (e.g., ACE2 for SARS-CoV-2, CD4/CCR5 for HIV, CD21 for EBV), enter cells, uncoat, replicate using host or viral machinery, assemble, and release progeny (lysis or budding). Cytopathic effects, immune-mediated damage (CD8+ T cell killing of infected cells), and immune evasion (antigenic variation, latency, downregulation of MHC I) determine pathogenicity.
Key clinical categories tested: - DNA viruses: HSV (vesicular lesions, latency in ganglia), VZV, CMV (owl's eye inclusions, congenital infection), EBV (infectious mononucleosis, Burkitt lymphoma), HBV (chronic hepatitis, HCC), adenovirus, parvovirus B19 (fifth disease, aplastic crisis). - RNA viruses: Influenza (segmented genome, antigenic shift/drift), HIV (retrovirus, CD4 depletion, reverse transcriptase), HCV (chronic hepatitis, cirrhosis), Rabies (Negri bodies), Rotavirus (pediatric diarrhea), Measles/Mumps/Rubella, Coronaviruses (SARS-CoV-2).
Diagnosis: PCR (gold standard for many), serology (IgM=acute, IgG=past/immunity), viral culture, antigen tests, Tzanck smear (HSV/VZV), biopsy findings (inclusion bodies - e.g., Cowdry A in HSV, owl's eye in CMV).
Treatment: Nucleoside/nucleotide analogs (acyclovir, tenofovir), reverse transcriptase inhibitors, protease inhibitors, integrase inhibitors, neuraminidase inhibitors (oseltamivir), interferons. Antivirals are pathogen mechanism-specific.
Immune response: Innate (interferons, NK cells) followed by adaptive (CD8+ cytotoxic T cells for intracellular killing, antibodies for extracellular neutralization). Latency (herpesviruses) and chronic infection (HBV, HCV, HIV) are key exam concepts.
High-yield associations: EBV-Burkitt/Hodgkin lymphoma, HPV-cervical/anal cancer, HBV/HCV-hepatocellular carcinoma, HTLV-1-adult T cell leukemia, HHV8-Kaposi sarcoma.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- Robbins Basic Pathology
- CDC/WHO Viral Disease Guidelines
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.