HIV — Pathogenesis, Staging and Complications

Human immunodeficiency virus (HIV) is a retrovirus that causes acquired immunodeficiency syndrome (AIDS) through progressive destruction of CD4+ T lymphocytes and the broader immune system. As of 2024, approximately 40 million people worldwide live with HIV, with nearly 1.3 million new infections annually and disproportionate burden in sub-Saharan Africa and among men who have sex with men (MSM). The virus exists on a continuum from acute primary infection through chronic asymptomatic infection to AIDS-defining illnesses, with CD4+ count serving as the primary determinant of immunosuppression severity and opportunistic infection risk. Understanding HIV pathogenesis and staging is critical for medical practice because antiretroviral therapy (ART) efficacy depends on early diagnosis, accurate CD4+ stratification, and recognition of immune reconstitution inflammatory syndrome (IRIS) during treatment initiation. For board examination purposes, candidates must distinguish the three stages of infection, apply current staging criteria incorporating both CD4+ count and clinical disease category, and recognize the major opportunistic infections and malignancies that define AIDS.

HIV's devastating clinical course stems from its unique capacity to directly target the adaptive immune system while simultaneously promoting chronic activation and dysfunction of both innate and adaptive immunity. The disease progresses through mechanistically distinct phases, each with characteristic virologic and immunologic features that drive clinical manifestations.

HIV initiates infection by binding the CD4 receptor and a coreceptor (CCR5 or CXCR4) on target cells, principally CD4+ T lymphocytes (specifically TH cells), but also macrophages, dendritic cells, and microglia. This dual-receptor requirement explains why individuals with CCR5-Δ32 deletion mutation (conferring loss of CCR5 function) remain resistant to R5-tropic strains, a principle exploited by maraviroc, a CCR5 antagonist. Following membrane fusion mediated by gp120/gp41 envelope proteins, the viral reverse transcriptase converts the single-stranded RNA genome into DNA within the cytoplasm—a process uniquely vulnerable to reverse transcriptase inhibitors. The newly synthesized DNA enters the nucleus where integrase catalyzes integration into the host chromosome, creating a persistent proviral genome that replicates with the host cell and cannot be eliminated by the immune system alone. This integration explains why HIV cannot be "cured" by immune reconstitution alone and necessitates lifelong ART in the vast majority of patients. The integrated provirus undergoes explosive replication once activated: a single infected CD4+ cell produces approximately 1,000 viral particles daily, with an estimated 10 billion virions produced system-wide per day during acute infection—a replication rate generating ~2,000 mutations daily, explaining HIV's extraordinary capacity for drug resistance development.

Direct cytotoxic destruction of infected CD4+ cells accounts for one component of immune loss: as reverse transcriptase synthesizes viral DNA, accumulating nucleotide analogs and abortive products trigger apoptosis via mitochondrial-mediated death pathways. However, direct killing of infected cells explains only partial CD4+ loss; additional mechanisms include bystander apoptosis (whereby uninfected CD4+ cells undergo programmed death through contact with infected cells via Fas-FasL interaction), antibody-dependent cellular cytotoxicity (ADCC) mediated by HIV envelope-specific antibodies binding uninfected cells, thymic involution reducing naïve CD4+ production, and lymphoid tissue fibrosis progressively destroying the architecture required for T cell regeneration. The thymus becomes fibrotic and dysfunctional, reducing naïve CD4+ T cell output, while lymph nodes undergo germinal center hyperplasia in early infection followed by progressive destruction and fibrosis in late disease. CD4+ count decline averages 50-100 cells/μL annually in untreated infection, though substantial individual variation exists; some long-term non-progressors (LTNPs) maintain normal CD4+ counts for decades despite persistent viremia through incompletely understood mechanisms involving robust CD8+ T cell responses or infection with attenuated viral strains.

Beyond absolute CD4+ depletion, HIV infection induces severe qualitative immune dysfunction characterized by chronic immune activation—paradoxically, despite progressive immunosuppression, untreated HIV patients manifest markedly elevated levels of circulating TNF-α, IL-6, IL-1β, and other pro-inflammatory cytokines, driving both non-specific immune activation and accelerated T cell apoptosis. Chronic viral antigen exposure drives exhaustion of CD8+ T cells, characterized by upregulation of PD-1 (programmed death receptor-1), TIM-3, and other inhibitory receptors, rendering these cells dysfunctional despite high numbers early in infection. Bacterial translocation from the damaged gastrointestinal mucosa contributes substantially to chronic immune activation: HIV selectively destroys Peyer's patches and mucosal-associated lymphoid tissue (MALT), disrupting the epithelial barrier and allowing microbial products (particularly lipopolysaccharide) to enter systemic circulation, continuously driving monocyte and dendritic cell activation. This mechanisms explains why HIV+ patients show persistently elevated D-dimer, C-reactive protein (CRP), IL-6, and TNF-α levels even after years of ART-mediated viral suppression—a phenomenon termed residual immune activation that contributes to accelerated aging and elevated cardiovascular risk. The gut epithelium in HIV infection shows marked tight junction dysfunction with increased apoptosis of enterocytes, reduced secretory IgA production, and depletion of Th17 cells (critical for mucosal immunity), all contributing to bacterial translocation and pathogenic immune activation.

During initial ART, as viral replication decreases and CD4+ counts recover, patients paradoxically experience immune reconstitution inflammatory syndrome (IRIS)—an exaggerated inflammatory response to opportunistic pathogens (OPs) or their antigens. Pathogenesis involves rapid expansion of previously suppressed, antigen-specific T cells encountering high pathogen antigen burdens before adequate pathogen clearance; the restored immune system mounts an excessive Th1-mediated inflammatory response causing severe, sometimes fatal organ inflammation. Paradoxical IRIS occurs when immune reconstitution causes clinical worsening of a currently treated OP (e.g., worsening tuberculous lymphadenitis despite anti-TB therapy), while unmasking IRIS represents undiagnosed OP presentation after immune recovery begins. This complication appears most frequently during the first 2-12 weeks of ART, particularly when CD4+ count is <50 cells/μL at ART initiation and rises rapidly, and in patients with high baseline viral loads and delayed diagnosis. The most common triggers are Mycobacterium tuberculosis, Mycobacterium avium complex (MAC), Cryptococcus neoformans, and Cytomegalovirus (CMV).

The relationship between plasma viral load (HIV RNA) and CD4+ count is imperfect but clinically crucial: viral load predicts rate of CD4+ decline and short-term (3-6 month) risk of opportunistic infection, while CD4+ count predicts immediate opportunistic infection risk. A plasma viral load >100,000 copies/mL predicts rapid CD4+ loss, while viral loads between 50,000-100,000 copies/mL carry intermediate risk. CD4+ count thresholds define specific OP risks: CD4+ <200 cells/μL confers risk for Pneumocystis jirovecii pneumonia (PCP), CD4+ <100 cells/μL for toxoplasmosis and cryptococcal meningitis, and CD4+ <50 cells/μL for CMV and Mycobacterium avium complex. Even after prolonged ART-mediated viral suppression, CD4+ recovery may be incomplete or delayed, particularly in patients starting ART with very low CD4+ counts; approximately 50% of patients starting ART with CD4+ <50 cells/μL achieve CD4+ >200 cells/μL after 12 months, and some never fully recover CD4+ function despite undetectable viremia.

HIV transmission occurs through three primary mechanisms: parenteral exposure to infected blood, sexual transmission with mucosal exposure to infectious secretions, and perinatal transmission from mother to infant. Understanding transmission risk enables identification of at-risk populations requiring testing and prevention strategies.

Receptive anal intercourse carries the highest per-act transmission probability (~1.4%), reflecting the fragility of rectal mucosa, high density of target lymphocytes in rectal tissue, and relative lack of protective secretory IgA. Receptive vaginal intercourse carries ~0.08% per-act transmission risk (higher during menses due to increased mucosal inflammation and target cell availability), while insertive vaginal intercourse risk is ~0.04% per-act. Insertive anal intercourse carries ~0.11% per-act risk, and receptive penile-oral intercourse carries approximately 0.04% risk. These per-act probabilities increase substantially in the presence of other sexually transmitted infections (STIs) causing mucosal inflammation, during acute HIV infection (when viral load peaks to 10⁶-10⁷ copies/mL in blood and substantially higher in genital secretions), and with undetectable viremia eliminated entirely through the "U=U" (undetectable equals untransmittable) principle—demonstrated in the PARTNER and PARTNER2 studies, which found zero HIV transmissions among serodiscordant couples where the HIV+ partner maintained viral load <200 copies/mL over median 16 month follow-up. Anal intercourse's elevated risk reflects tears in the thin, non-keratinized rectal epithelium, high concentration of Langerhans cells and macrophages expressing CCR5, and increased local inflammatory cytokines. Vaginal/genital inflammation from bacterial vaginosis, trichomoniasis, herpes simplex virus (HSV), or chlamydia increases transmission risk 2-5 fold by recruiting CD4+ target cells and disrupting epithelial barriers.

Needle/syringe sharing among people who inject drugs (PWID) carries approximately 0.63% per-act transmission risk and accounts for persistent HIV prevalence in this population globally, with higher transmission when sharing equipment during acute infection or with visibly bloody injections. Occupational needlestick exposure from HIV-positive patient carries ~0.3% per-exposure transmission risk; risk increases with deeper penetrating wounds, visible blood on device, needle placed directly in patient's blood vessel, and patient in terminal illness stage (reflecting higher viral load). Transfusion of infected blood carries >90% transmission probability, explaining why blood screening is epidemiologically crucial. Organ transplantation from HIV+ donors to HIV+ recipients (increasingly performed with excellent outcomes) carries minimal additional transmission risk. Body fluid exposure to mucous membranes or non-intact skin (healthcare worker splash exposure) carries substantially lower risk (~0.09%) than parenteral routes.

Mother-to-child transmission (MTCT) occurs via three mechanisms: intrauterine transmission (across placental barrier, accounting for ~5-10% of MTCT), intrapartum transmission during labor/delivery through contact with infected blood and genital secretions (accounting for ~70-90% of MTCT), and postpartum transmission via breast milk in resource-limited settings (accounting for additional 15-45% transmission risk if breastfeeding). Without intervention, MTCT rates reach 15-45%; antenatal and intrapartum ART reduces transmission to <1-2% in resource-rich settings and <5% in resource-limited settings when combined with infant nevirapine prophylaxis, elective cesarean delivery (if maternal viral load >1,000 copies/mL), and formula feeding instead of breastfeeding. Intrauterine transmission risk correlates with placental inflammation, placental insufficiency, and maternal plasma viral load >100,000 copies/mL.

Men who have sex with men (MSM) represent approximately 40-45% of new HIV diagnoses in the United States despite comprising 2-4% of the population, reflecting higher per-act transmission probability of receptive anal intercourse and, historically, lower awareness of prevention strategies. Sex workers and their clients experience elevated transmission risk through high-frequency partner exposure and biological/economic vulnerabilities. Incarcerated individuals show elevated HIV prevalence due to concentrated populations of PWID, limited condom access, and sexual coercion. Transgender women show particularly elevated HIV prevalence globally (approximately 27% in US CDC surveillance data, versus 0.3% in general US population), driven by combination of receptive anal intercourse, sex work, discrimination limiting healthcare access, and psychosocial stressors. Healthcare workers in resource-limited settings with high patient loads, frequent needle/sharps exposure, and inconsistent infection control practices face elevated occupational risk, though with appropriate precautions, needlestick injury risk is extremely low (0.3%).

HIV's clinical manifestations span three distinct stages—acute primary infection, chronic asymptomatic infection, and AIDS—with symptomatology driven by viral load, CD4+ count, and comorbid opportunistic infections or malignancies.

Acute primary infection occurs in 50-90% of newly infected individuals within 2-4 weeks of exposure and coincides with explosive viral replication and plasma viremia reaching 10⁶-10⁷ copies/mL—the highest viral loads during the entire disease course. Patients present with flu-like illness including high fever (typically 38.5-40°C), profound myalgia and arthralgia (particularly affecting knees and shoulders), headache (sometimes meningeal with photophobia), and generalized malaise. The characteristic rash appears in 40-80% of patients as erythematous, maculopapular rash involving trunk and face (particularly sparing palms and soles), usually non-pruritic, appearing 3-6 days after fever onset and persisting 5-10 days. Lymphadenopathy occurs in 70-90% (cervical, axillary, inguinal nodes, often symmetric and tender), pharyngitis with oral ulcers (specifically aphthous ulcers on buccal mucosa, palate, or tongue), and occasionally esophageal ulcers causing severe dysphagia. GI symptoms are prevalent: diarrhea, nausea, vomiting, and abdominal pain reflect CD4+ depletion of gut-associated lymphoid tissue (GALT) and enterocyte apoptosis. Neurologic manifestations include aseptic meningitis (pleocytosis with lymphocytic predominance, elevated protein, normal glucose in CSF, negative bacterial cultures), peripheral neuropathy, and rarely Guillain-Barré syndrome. Thrush (oral candidiasis with white plaques on tongue/buccal mucosa) can occur despite typically preserved CD4+ counts during acute infection, reflecting the overwhelming viral load and transient localized immune impairment. Hepatosplenomegaly appears in 10-15% due to direct viral invasion and mononuclear cell infiltration. Importantly, acute primary infection is highly transmissible due to peak viremia and patients' unawareness of infection status; many secondary transmissions occur during this window. The diagnosis is frequently missed clinically because symptoms mimic infectious mononucleosis, influenza, or other viral exanthems—a critical board trap is attributing febrile illness with lymphadenopathy and rash to EBV without considering acute HIV.

Following acute infection, patients typically enter a prolonged asymptomatic stage lasting years to decades (median 8-10 years without ART), during which CD4+ count gradually declines but remains above threshold for opportunistic infection. During this period, most patients are completely asymptomatic, with no physical exam findings, though baseline laboratory abnormalities develop: elevated LDH (from lymphocyte turnover), elevated inflammatory markers (CRP, D-dimer), leukopenia and thrombocytopenia from direct HIV suppression of bone marrow, and anemia from combined cytokine effects and nutritional deficiency. The viral load continues to fluctuate but typically stabilizes between 1,000-100,000 copies/mL (the "set point") in untreated infection; set point is established within first

Immediate priorities: The DHHS/NIH Panel on Antiretroviral Guidelines recommends antiretroviral therapy (ART) for every person with HIV regardless of CD4+ count, started as soon as possible — including same-day initiation — because early suppression preserves CD4+ cells, reduces inflammation, and renders the patient non-infectious sexually (U=U). Do not wait for the genotype to return; adjust later if resistance is found.

Escalation: Confirmed virologic failure requires an adherence assessment plus genotypic resistance testing on therapy. Construct a new regimen with at least two (preferably three) fully active drugs — never add a single active agent to a failing regimen. Salvage options include second-generation INSTIs, boosted darunavir, and agents such as the attachment inhibitor fostemsavir or the CD4-directed monoclonal ibalizumab in multidrug-resistant disease.

Prophylaxis and prevention (CDC/NIH/IDSA opportunistic infection guidelines): TMP-SMX when CD4+ <200 cells/μL (covers Pneumocystis and, with positive Toxoplasma IgG, toxoplasmosis), discontinued after sustained CD4+ recovery on ART. Pre-exposure prophylaxis (tenofovir/emtricitabine or long-acting cabotegravir) carries a USPSTF grade A recommendation; post-exposure prophylaxis is a 3-drug regimen begun within 72 hours for 28 days.