49. Retroviridae – the causative agent of AIDS (HIV).

I. General Characteristics

• Family: Retroviridae | Genus: Lentivirus
• Human Immunodeficiency Virus (HIV) includes:
• HIV-1: Most widespread globally; more virulent
• HIV-2: Less transmissible, slower progression to AIDS; mostly in West Africa
• Enveloped, spherical virus, ~100–120 nm in size
• Genome: 2 copies of (+)ssRNA, diploid, reverse transcribed into DNA
• Contains enzyme reverse transcriptase, integrase, protease
• Transmission via:
• Sexual contact
• Blood transfusion/needle sharing
• Vertical (mother-to-child): transplacental, perinatal, breastfeeding

III. Replication Cycle (Key to Pathogenesis)

1. Attachment: gp120 binds to CD4 on T-helper cells, macrophages, dendritic cells

• Requires co-receptor: CCR5 (early), CXCR4 (late-stage)

2. Fusion: via gp41 → viral entry into cell
3. Reverse Transcription: RNA → cDNA via reverse transcriptase
4. Integration: cDNA integrated into host genome by integrase
5. Latency or active replication (depending on immune state)
6. Budding and maturation: new virions released, matured by protease

IV. Pathogenesis

• HIV targets and destroys CD4+ T cells → gradual immune system collapse
• Early stage: high viremia, transient drop in CD4+ cells, flu-like illness
• Clinical latency: low viremia, stable CD4 count, virus persists in lymphoid tissue
• AIDS: CD4 < 200 cells/μL or presence of AIDS-defining illnesses (e.g., PCP, Kaposi sarcoma)