Chapter 26: Retroviridae, HIV, and AIDS

The Retroviridae family encompasses viruses with a fundamentally distinct genetic strategy that inverts the conventional flow of biological information from DNA to RNA. This chapter elucidates how retroviruses, particularly HIV, employ reverse transcriptase to synthesize DNA from their RNA genome and permanently embed this genetic material into host chromosomes, establishing persistent infection. The discussion differentiates between acute transforming retroviruses that carry captured cellular proto-oncogenes for rapid malignant transformation and non-acute transforming retroviruses that induce cancer through insertional mutagenesis, activating endogenous oncogenic pathways. HIV structure is examined in detail, highlighting the lipid bilayer envelope decorated with gp120 and gp41 surface glycoproteins essential for cellular recognition, the p24 protein forming the internal capsid, and the two copies of RNA genome encoding critical regulatory and structural genes. The viral replication cycle proceeds through sequential stages: initial attachment to CD4 molecules and chemokine coreceptors on target cells including T-helper lymphocytes and macrophages, membrane fusion, reverse transcription in the cytoplasm, nuclear entry of the DNA copy, chromosomal integration, and release of progeny virions through budding. Clinical progression unfolds across three phases beginning with acute infection symptoms, transitioning through an extended clinical latency period characterized by paradoxical viral proliferation within lymphoid tissues despite minimal symptoms, and culminating in immunological collapse when CD4 counts fall below critical thresholds. AIDS diagnosis reflects this collapse through profound immunosuppression enabling a constellation of opportunistic complications including Pneumocystis jirovecii and Cryptococcus infections, Cytomegalovirus reactivation causing retinal disease, Toxoplasma encephalitis, and disseminated Mycobacterium infections. The chapter addresses associated malignancies such as Kaposi sarcoma and B-cell lymphomas arising from immunosuppression and viral coinfection. Treatment approaches center on highly active antiretroviral therapy combining multiple drug classes targeting reverse transcriptase and viral proteases, designed to suppress replication below detectable limits and restore immune function, though viral evolution and drug resistance present ongoing clinical challenges requiring careful monitoring.