Capsid symmetry reflects genetic economy: a small genome can only encode a limited repertoire of proteins, so viruses solve the "coat problem" by using many identical copies of one or a few proteins (capsomeres) that self-assemble via defined protein-protein contacts — icosahedral symmetry (20 triangular faces) is the most efficient closed-shell arrangement for this purpose, explaining its prevalence.

The Baltimore classification system groups viruses by the pathway from genome to mRNA: Class I (dsDNA), II (ssDNA), III (dsRNA), IV (+ssRNA, acts directly as mRNA), V (−ssRNA, requires viral RNA-dependent RNA polymerase to make mRNA), VI (retroviruses, ssRNA reverse-transcribed to dsDNA), VII (dsDNA viruses replicating through an RNA intermediate, e.g., Hepatitis B).

Attachment specificity is determined by a lock-and-key fit between a viral surface protein and a host receptor — this receptor choice is a major determinant of tissue tropism and host range (e.g., HIV's gp120 requires both CD4 and a chemokine co-receptor CCR5/CXCR4; influenza hemagglutinin binds sialic acid residues, and the specific linkage type — alpha-2,6 in humans vs. alpha-2,3 in birds — underlies species barriers).

Lysogenic induction is classically triggered by the bacterial SOS response: DNA damage (e.g., from UV light or certain antibiotics) activates RecA, which promotes autocleavage of the phage repressor protein (analogous to LexA cleavage in the bacterial SOS system), de-repressing lytic genes and triggering prophage excision.

RNA-dependent RNA polymerases lack 3'→5' exonuclease proofreading activity (with the notable exception of some coronaviruses, which have a proofreading exonuclease — nsp14 — contributing to their comparatively larger, more stable genomes). This absence of proofreading in most RNA viruses produces mutation rates roughly a million-fold higher than DNA-based organisms, generating quasispecies — a diverse population cloud of closely related genomes within a single infected host.

HIV resistance mutations are catalogued by their position in reverse transcriptase or protease; some mutations confer resistance to an entire drug class (e.g., the M184V reverse transcriptase mutation confers high-level lamivudine resistance but, notably, also modestly resensitizes the virus to zidovudine — a clinically relevant interaction when selecting salvage regimens).

• EBV's latency is maintained via expression of latent membrane proteins (LMP1 acts as a constitutively active CD40 mimic, driving B-cell proliferation) — this same oncogenic mechanism underlies EBV's association with Burkitt lymphoma (classically involving a c-myc translocation, t(8;14)) and nasopharyngeal carcinoma.
• CMV mononucleosis classically produces a heterophile-antibody-NEGATIVE mononucleosis-like syndrome, distinguishing it from EBV's heterophile-positive (Monospot-positive) presentation.

• HIV entry requires sequential binding: gp120 first engages CD4, inducing a conformational change that exposes the co-receptor binding site (CCR5 for macrophage-tropic "R5" strains, typically transmitted strains; CXCR4 for T-cell-tropic "X4" strains, often emerging later in disease). gp41 then mediates membrane fusion.
• Individuals homozygous for the CCR5-Δ32 deletion lack functional CCR5 and are highly resistant to R5-strain HIV infection — the molecular basis of the "Berlin patient" and similar functional cure cases achieved via CCR5-Δ32 homozygous donor stem cell transplantation.