Virus = nucleic acid + capsid (± envelope). Naked viruses survive environment better; enveloped are easier to kill.
Viral Structure
What every virus is made of — and why it matters clinically
Capsid: protein coat protecting nucleic acid. Enveloped viruses (HIV, influenza, herpes): lipid membrane from host cell — destroyed by soap/alcohol. Naked (non-enveloped) viruses (adenovirus, norovirus, poliovirus): resistant to drying and stomach acid — fecal-oral transmission.
Baltimore Classification
Baltimore classification: 7 groups by genome type and replication strategy — "How does the virus make mRNA?"
Baltimore Classification
The universal system for classifying viruses by their genetic strategy
Group I: dsDNA (herpes, adenovirus). Group II: ssDNA (parvoviruses). Group III: dsRNA (rotavirus). Group IV: +ssRNA (poliovirus, hepatitis C). Group V: -ssRNA (influenza, rabies). Group VI: ssRNA retroviruses (HIV). Group VII: dsDNA retroviruses (hepatitis B). Key: +strand RNA = can be directly translated as mRNA.
+ssRNA
Acts as mRNA directly — poliovirus, HCV
-ssRNA
Needs RNA-dep RNA polymerase — influenza, rabies
Retrovirus
RNA → DNA via reverse transcriptase — HIV
HIV Replication
HIV targets CD4+ T cells. RT makes DNA from RNA. Integrase inserts into host genome. Protease matures virions.
HIV Replication Steps
HIV life cycle — and which step each drug class blocks
HIV binds CD4 + CCR5/CXCR4 coreceptors. Reverse transcriptase (RT) converts RNA → DNA (error-prone → mutations → resistance). Integrase incorporates DNA into host chromosome (latency). Protease cleaves polyproteins into functional components. Drug targets: NRTIs/NNRTIs (RT), integrase inhibitors, protease inhibitors, fusion inhibitors.
Herpesvirus Latency
Herpesviruses: establish latency, reactivate when immunity drops. "Herpes never leaves."
Herpesvirus Latency
Why herpes infections recur throughout life
HSV-1/2: latent in sensory ganglia — reactivates as cold sores or genital lesions. VZV: primary = chickenpox; latency in dorsal root ganglia; reactivation = shingles (zoster). EBV: latent in B lymphocytes — reactivates in immunocompromised patients (lymphoma). CMV: latent in myeloid cells — dangerous in transplant recipients.