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Viral pathogenesis involves attachment to host receptors, entry (fusion, endocytosis, or injection), gene expression, replication, and egress. Virulence is determined by viral gene products (toxins, immune evasion), host factors (innate immunity, age), and epidemiological context. Cytopathic effects (cell lysis, syncytia, inclusion bodies) are hallmarks of viral infection.
From your study of viral attachment glycoproteins and host-pathogen interactions, you know that viruses cannot replicate on their own — they must hijack a host cell's machinery. Viral pathogenesis is the study of how this hijacking unfolds step by step and how it produces disease. The process follows a stereotyped sequence: attachment, entry, gene expression and replication, assembly, and egress. Each step presents both a vulnerability the host immune system can exploit and a point where the virus has evolved countermeasures.
Attachment is the first and most specific step. The viral surface proteins you studied — glycoproteins like HIV's gp120 or influenza's hemagglutinin — bind to particular receptors on host cells. This receptor specificity is what determines tropism: which cell types, tissues, and even species a virus can infect. HIV targets CD4⁺ T cells because gp120 binds the CD4 receptor; rabies virus targets neurons because its glycoprotein G binds the nicotinic acetylcholine receptor. After attachment, entry occurs by one of three general mechanisms: direct fusion of viral and host membranes (HIV), receptor-mediated endocytosis followed by pH-triggered fusion in the endosome (influenza), or injection of the genome through the cell wall (bacteriophages). The entry route matters clinically because it determines which antiviral strategies can block infection at the earliest stage.
Once inside, the virus redirects host ribosomes, polymerases, and metabolic resources to produce viral proteins and copy the viral genome. This is where virulence factors come into play. Some viruses encode proteins that shut down host protein synthesis (poliovirus cleaves eIF4G), redirect immune signaling (Ebola's VP35 blocks interferon induction), or prevent apoptosis so the infected cell survives long enough to produce more virions. The observable damage to infected cells — cytopathic effects — takes several characteristic forms: lysis (the cell bursts, releasing new virions), syncytia formation (viral fusion proteins on the cell surface cause neighboring cells to merge into giant multinucleate masses, as seen with measles and RSV), and inclusion bodies (dense aggregates of viral components visible under the microscope, like the Negri bodies diagnostic of rabies).
The outcome of infection depends on the balance between viral offense and host defense. A virus that replicates explosively and lyses cells causes acute disease (influenza, norovirus), while one that integrates into the genome or persists in a latent state causes chronic or recurrent disease (HIV, herpes simplex). The host's innate immune response — interferon signaling, natural killer cells, inflammation — acts as the first line of defense, and many of the most dangerous viruses are dangerous precisely because they have evolved ways to evade or suppress this response. Understanding pathogenesis as a dynamic interplay between viral strategy and host counterstrategy, rather than a simple cause-and-effect, is the key insight that connects molecular virology to clinical medicine.