Infection Mechanisms

A virus outside a cell is inert. It cannot replicate, cannot respond to its environment, cannot do anything. It is only within a host cell that the viral program executes — and the path from "outside the cell" to "producing thousands of new virions" involves a series of precisely orchestrated molecular steps.

Understanding infection mechanisms is not just virology trivia. Every step of the viral lifecycle is a potential drug target. Most antivirals in use today block one or more of these steps: entry inhibitors, reverse transcriptase inhibitors, protease inhibitors, polymerase inhibitors, neuraminidase inhibitors. Mechanistic understanding directly enables therapeutic intervention.

The Viral Replication Cycle

Despite enormous diversity, all viruses follow the same basic program:

1. Attachment   — virus binds host cell surface receptor
2. Entry        — virus gets genome into the cytoplasm
3. Replication  — viral genome is copied
4. Transcription/Translation — viral proteins are synthesized
5. Assembly     — new virions are built
6. Release      — virions exit the cell

Let's examine each step in detail using specific, well-characterized viruses as examples.

Step 1: Attachment — Finding the Right Host

Viruses don't infect all cells indiscriminately. Infection specificity (tropism) is determined largely by which receptor a virus uses for attachment.

HIV

HIV's surface protein gp120 binds the CD4 receptor on T helper cells (and monocytes/macrophages). But CD4 binding alone isn't sufficient — gp120 then binds a co-receptor, either CCR5 or CXCR4 (chemokine receptors), triggering the structural changes needed for membrane fusion.

Why this matters clinically: Individuals with a homozygous deletion in CCR5 (CCR5Δ32) are almost completely resistant to HIV-1 infection. This led to the development of the first entry inhibitor: maraviroc, a CCR5 antagonist. It also inspired the Berlin Patient experiment — using a CCR5Δ32 bone marrow transplant to cure HIV in Timothy Ray Brown.

SARS-CoV-2

SARS-CoV-2's spike protein binds ACE2 (angiotensin-converting enzyme 2) on respiratory epithelial cells. ACE2 expression is highest in the lungs, gut, and heart — explaining the tropism of COVID-19. The spike protein must also be cleaved by a host protease (TMPRSS2 or furin) for activation.

Tracking spike protein mutations (D614G, Delta N501Y, Omicron BA.1 etc.) became the core work of COVID-19 genomic surveillance — mutations in the receptor-binding domain affect ACE2 affinity and immune evasion.

Influenza

Influenza's hemagglutinin (HA) binds sialic acid residues on cell surface glycoproteins. Human-tropic influenza preferentially binds α-2,6-linked sialic acids (abundant in the upper respiratory tract); avian-tropic influenza binds α-2,3-linked sialic acids (in the lower respiratory tract and gut). This difference in sialic acid linkage is part of why most avian influenza strains don't efficiently infect humans.

Step 2: Entry — Getting the Genome In

After receptor binding, the virus must deliver its genome to the cytoplasm (or nucleus, for DNA viruses). Two main strategies:

Membrane Fusion (Enveloped Viruses)

After receptor binding, the viral envelope fuses with either:

• The plasma membrane directly (HIV, SARS-CoV-2 with TMPRSS2 cleavage)
• The endosomal membrane after endocytosis (influenza, SARS-CoV-2 via cathepsin pathway)

For influenza:

1. Virus is endocytosed
2. The endosome acidifies (pH drops to ~5)
3. Low pH triggers conformational change in HA, exposing the fusion peptide
4. Fusion peptide inserts into the endosomal membrane
5. Membranes merge, releasing the viral genome segments into the cytoplasm

Drug target: Amantadine/rimantadine block the M2 ion channel that acidifies the influenza virion interior — needed for genome uncoating. Resistance mutations (most circulating strains are now resistant) illustrate how antivirals drive viral evolution.

Capsid Penetration (Non-Enveloped Viruses)

Non-enveloped viruses face a harder problem: no lipid membrane to fuse with. They use various mechanisms:

• Poliovirus: receptor-triggered conformational change creates a pore in the endosomal membrane
• Adenovirus: escapes the endosome by disrupting the endosomal membrane with its penton proteins
• Parvovirus: phospholipase activity in the capsid disrupts the endosomal membrane

Step 3 & 4: Replication and Protein Synthesis

Replication strategy varies dramatically by genome type:

Positive-Sense RNA Viruses (SARS-CoV-2, Poliovirus, Dengue)

1. The +ssRNA genome is immediately translated by host ribosomes → produces a large polyprotein
2. The polyprotein is cleaved by viral proteases into individual proteins, including the RNA-dependent RNA polymerase (RdRp)
3. The RdRp makes a complementary negative-sense strand
4. The negative-sense strand serves as template for making thousands of new positive-sense genome copies
5. Subgenomic RNAs are made from internal promoters → translate structural proteins

SARS-CoV-2's RdRp (nsp12/nsp7/nsp8 complex) is the target of remdesivir and molnupiravir. The viral proteases (Mpro/3CLpro and PLpro) are targeted by nirmatrelvir (in Paxlovid).

Retroviruses (HIV)

HIV's replication requires reverse transcription — a step unique to this class:

1. Viral RNA genome is reverse-transcribed to DNA by reverse transcriptase (RT)
2. The RNA template is degraded by RT's RNase H activity
3. The second DNA strand is synthesized, producing double-stranded DNA
4. The integrase enzyme integrates the viral DNA into a host chromosome — producing the provirus
5. The provirus is transcribed by host RNA Pol II → viral mRNAs and genomic RNA
6. Viral proteins are translated; Gag polyprotein is cleaved by HIV protease during budding

HIV antiretroviral targets: NRTI/NNRTI inhibitors target RT; protease inhibitors target HIV protease; integrase inhibitors (raltegravir, dolutegravir) target integrase. Modern antiretroviral therapy (ART) combines at least two drug classes to prevent resistance.

DNA Viruses (Herpesviruses)

DNA viruses generally replicate in the nucleus, using a combination of host and viral polymerases:

1. Viral DNA enters the nucleus
2. Host RNA Pol II transcribes immediate-early genes (regulatory)
3. Immediate-early proteins activate early gene transcription
4. Early proteins include viral DNA polymerase and other replication factors
5. Viral DNA is replicated by the viral DNA polymerase
6. Late genes are transcribed → structural proteins
7. Capsids assemble in the nucleus

Herpesviruses establish latency: after acute infection, viral DNA persists in neurons (HSV) or B cells (EBV) as a circular episome, with only a few latency-associated transcripts expressed. Stress, immunosuppression, or UV light can reactivate the virus to productive replication. Acyclovir targets the viral thymidine kinase and DNA polymerase — it's incorporated into viral DNA and terminates replication, but it doesn't eliminate latent virus.

Step 5: Assembly — Building New Virions

Viral proteins and genomic RNA/DNA are assembled into new virions in the cytoplasm (RNA viruses, some DNA viruses) or nucleus (herpesviruses).

Packaging specificity: The genome must be selectively packaged, not random cellular RNA. Packaging signals — specific RNA structures or sequences — are recognized by capsid proteins. HIV packages its genome via the Ψ (psi) packaging signal near the 5' end; mutations in Ψ prevent packaging.

Protein processing: Many viruses produce polyproteins that must be cleaved into functional subunits during assembly. HIV's Gag-Pol polyprotein is cleaved by the HIV protease as the new virion buds off. Protease inhibitors block this cleavage → immature, non-infectious virions are produced.

Step 6: Release — Propagating the Infection

Viruses exit infected cells either by:

Budding (enveloped viruses): Viral proteins are inserted into the host membrane; the capsid associates with these membrane patches and buds outward, acquiring its envelope. The virus is released without killing the cell. HIV and influenza use this strategy.

For influenza, the neuraminidase (NA) enzyme cleaves sialic acid from host cell surfaces, releasing newly budded virions that would otherwise stick to the source cell. Oseltamivir (Tamiflu) and zanamivir are NA inhibitors — they prevent virion release and reduce symptom severity.

Lysis (non-enveloped viruses, and some enveloped): The cell is destroyed, releasing virions. Poliovirus, adenovirus, and many bacteriophages lyse their host cells.

Transcytosis: Some viruses cross epithelial barriers by entering one side of a polarized cell and exiting the other — used for systemic spread.

Viral Fitness and Drug Resistance

Viruses, especially RNA viruses, mutate rapidly because their polymerases lack proofreading. HIV generates ~10¹⁰ virions per day in an untreated patient, with a mutation rate of ~3×10⁻⁵ per base per replication cycle. Given a genome of ~10 kb, virtually every possible single point mutation is generated multiple times per day.

This means:

• Resistance mutations preexist before treatment in the viral quasispecies
• Any single-drug treatment selects for pre-existing resistant variants within days
• Combination therapy (ART) is required — the probability of pre-existing resistance to three drugs simultaneously is astronomically low

This evolutionary dynamic — rapid mutation + selection — is also how SARS-CoV-2 generated successive variants (Alpha, Delta, Omicron) and why influenza vaccines must be reformulated annually.

Understanding viral replication mechanisms is not just mechanistic knowledge — it provides the framework for predicting how viruses evolve under drug or immune pressure, which is core work in viral genomics.