Opportunistic Infections in HIV/AIDS

• CD4+ T-cell-mediated immunity collapse: HIV specifically targets CD4+ T cells (through CD4 receptor and CCR5/CXCR4 co-receptors), leading to quantitative and qualitative defects in cell-mediated immunity. As CD4 counts decline below 200 cells/μL, the ability to mount Th1 responses (necessary for intracellular pathogens) deteriorates catastrophically. This explains why OIs like Mycobacterium avium complex (MAC), Toxoplasma gondii, and Cryptosporidium become prevalent—these organisms require intact T-cell responses for control. The loss of CD4+ T cells results in impaired cytokine production (IL-2, IFN-γ, TNF-α), reduced activation of macrophages, and failure of antigen presentation, allowing intracellular parasites to replicate unchecked.

• Loss of mucosal barrier immunity and local antimicrobial defenses: HIV-induced damage to gut-associated lymphoid tissue (GALT) disrupts the intestinal epithelial barrier through loss of intraepithelial lymphocytes and regulatory T cells. This permits bacterial translocation and dissemination of organisms typically contained to mucosal surfaces. Additionally, reduced secretory IgA production, impaired neutrophil function, and diminished natural killer (NK) cell activity compromise innate immunity, predisposing to infections by Salmonella, Shigella, and fungal organisms. The loss of GALT immunity is particularly significant in the pathogenesis of chronic diarrhea in advanced HIV disease.

• Macrophage dysfunction and impaired phagocytosis: Although CD4 counts may be preserved in some macrophage-tropic infections, HIV-infected macrophages themselves display reduced microbicidal activity due to decreased respiratory burst (ROS production) and impaired phagolysosomal fusion. This mechanism is central to Mycobacterium tuberculosis dissemination and MAC pathogenesis. Macrophages become HIV reservoirs, perpetuating infection and spreading organisms systemically.

• Impaired B-cell function and antibody responses: Beyond CD4 loss, HIV causes quantitative and qualitative B-cell dysfunction. Hypergammaglobulinemia occurs paradoxically alongside inability to generate protective antibodies to new antigens. This dichotomy explains why polysaccharide vaccine responses are poor in advanced HIV disease but pre-existing antibodies (e.g., to measles) may persist at subprotective levels. This contributes to severe bacterial infections (Streptococcus pneumoniae, Haemophilus influenzae) despite preserved B-cell numbers.

• Chronic immune activation and metabolic exhaustion: Persistent antigenic stimulation (both from HIV and colonizing pathogens) leads to T-cell exhaustion with upregulation of inhibitory molecules (PD-1, CTLA-4) and shift toward Th2 responses. Elevated IL-6 and TNF-α contribute to cachexia and metabolic dysfunction. Additionally, reduced thymic output (from HIV-induced thymic involution) limits replenishment of T-cell repertoire, and increased apoptosis via Fas-FasL interactions further decimates CD4 populations.

• Breakdown of commensal control mechanisms: The normal flora of skin, mouth, and GI tract are controlled by both physical barriers and local immunity. Loss of Langerhans cells in skin, reduced salivary IgA, and decreased local cytokine production permit overgrowth of Candida albicans, Herpes simplex virus, and Epstein-Barr virus (EBV). The oral-esophageal candidiasis pathway exemplifies this: reduced CD4-mediated immunity → loss of control of commensal Candida → pseudomembrane formation → esophageal involvement with dysphagia and odynophagia.

• CD4+ T-cell count <200 cells/μL (primary immunologic driver): This is the defining threshold below which OI risk increases exponentially. The relationship is often quantitative: CD4 <200 cells/μL confers risk for multiple OIs; CD4 <100 cells/μL for CMV and toxoplasmosis; CD4 <50 cells/μL for MAC and CMV retinitis. Notably, baseline CD4 count at ART initiation and the rate of CD4 decline (velocity) influence OI risk—rapid decliners have higher risk at any given CD4 level. Prior opportunistic infections and the presence of chronic immune activation (high HIV RNA >100,000 copies/mL) independently increase risk.

• Geographic location and epidemiologic exposure: Coccidioides immitis is endemic to the southwestern United States and northern Mexico; Histoplasma capsulatum to Ohio and Mississippi River valleys; and Blastomyces dermatitidis to similar regions. Tuberculosis prevalence varies globally, with rates >25% in some developing nations versus <1% in developed countries. Toxoplasmosis prevalence correlates with seroprevalence of T. gondii (low in Northern Europe, high in France, Africa, and Latin America). Travel and migration history are critical—a patient from sub-Saharan Africa with CD4 <100 is at high risk for TB and cryptococcal meningitis. Environmental exposures (bird droppings for histoplasmosis and cryptococcosis, cat feces for toxoplasmosis) become clinically relevant only in the setting of severe immunosuppression.

• Prior history of the specific OI or low-level latent infection reactivation: Most OIs represent reactivation of latent infections acquired earlier when immunity was intact. Mycobacterium tuberculosis remains latent in granulomas until CD4 recovery fails; Toxoplasma cysts persist in brain tissue; Cryptosporidium may establish chronic infection. Patients with documented toxo seropositivity have ~30% risk of developing toxoplasmic encephalitis if CD4 drops below 100 without prophylaxis. This reactivation pattern explains why geographic epidemiology matters: without prior exposure, reactivation cannot occur.

• Lack of effective antiretroviral therapy: Virologic failure (detectable viral load despite ART) or non-adherence leading to subtherapeutic drug levels results in continued CD4 decline and OI development. Patients naïve to ART with CD4 <50 cells/μL have incidence of MAC of ~50/100 patient-years without prophylaxis. Even patients on suboptimal regimens with CD4 slowly increasing (e.g., CD4 100-200) remain at elevated OI risk until CD4 consistently exceeds protective thresholds for >3 months.

• Specific co-infections and comorbidities: Patients with active tuberculosis are at increased risk for other mycobacterial infections (MAC) due to overlapping immunologic defects. Hepatitis C coinfection may accelerate CD4 decline. Herpes simplex virus seropositivity confers risk for disseminated HSV; CMV seropositivity for CMV disease. Nutritional deficiencies (vitamin A, zinc) impair immune reconstitution and increase OI risk.

• Suboptimal or delayed immune reconstitution: Patients initiating ART with CD4 <50 cells/μL may experience delayed CD4 recovery (some gain <25 cells/μL in first year), prolonging vulnerability to OIs. Poor adherence, viral escape, or "blunted responders" (genetic or acquired factors limiting CD4 recovery) perpetuate risk. Additionally, the immune system requires time to recover antigenic memory, explaining why even CD4 >200 in first weeks of ART doesn't immediately eliminate OI risk.

Pneumocystis jirovecii pneumonia (PCP)

• Dyspnea on exertion and progressive dyspnea at rest: Caused by diffuse alveolar-interstitial inflammation with foamy exudate containing organisms and cellular debris. Hypoxemia often exceeds radiographic findings (PaO2 may be severely reduced with only subtle CXR findings). Patients often report weeks of progressive symptoms before seeking care, with dry cough preceding dyspnea. Tachypnea (often >30 breaths/min) is characteristic. Chest tightness and orthopnea may develop.
• Mild or absent fever: Unlike bacterial pneumonia, fever is often absent or low-grade (distinguishing feature). This reflects the fungal etiology and the lack of neutrophilic response in profoundly immunosuppressed patients.
• Examination findings: Crackles may be absent despite significant pneumonia; decreased O2 saturation with ambulation (desaturation with exercise testing is classic). Wheezing is uncommon but may occur.

Tuberculosis

• Cough, fever, night sweats, and weight loss: TB presents in two immunologic phases: CD4 >200 typically causes upper lobe cavitary disease (similar to immunocompetent TB), while CD4 <50 causes lower lobe, middle lobe, or miliary disease with lymphadenitis and minimal cavitation. Symptoms develop insidiously over weeks to months. Night sweats may be drenching, leading to cachexia.
• Disseminated disease in advanced immunosuppression: When CD4 <50, extrapulmonary TB occurs in 50-80% of patients, manifesting as lymphadenitis (particularly mediastinal with hilum enlargement), hepatosplenomegaly, CNS TB, TB meningitis, or bacteremia.
• Examination: Lymphadenopathy (painless, rubbery, often bilateral), hepatosplenomegaly, wasting.

Cryptococcal meningitis

• Headache, fever, and meningeal signs (absent or minimal): Cryptococcal meningitis develops subacutely over 1-4 weeks with insidious onset of headache, often mild, sometimes without fever. The paucity of meningeal signs (neck stiffness, Kernig's, Brudzinski's) is striking and reflects the minimal cerebrospinal fluid (CSF) pleocytosis typical of cryptococcal disease in CD4 <50 (lymphocytic, often <50 cells/μL, sometimes <20). This contrasts sharply with bacterial meningitis and reflects the inability to mount inflammatory response.
• Subtle neurologic findings: Altered mental status, confusion, or personality changes may be the only presenting feature. Cranial nerve palsies are relatively uncommon.
• Examination pearls: Fever may be absent or low-grade; meningeal signs often absent despite positive CSF cultures. This leads to diagnostic delays and is a classic USMLE trap.

Toxoplasmic encephalitis

• Focal neurologic deficits (hemiparesis, speech abnormality, seizures): Toxoplasmosis typically presents with one or more focal brain lesions causing hemiparesis, homonymous hemianopia, aphasia, ataxia, or seizures. Headache and fever often precede focal findings. Symptoms develop over days to weeks. Mass effect from lesions can cause increased intracranial pressure with headache and altered mental status.
• Multiple ring-enhancing lesions on imaging: Multiplicity is key—solitary lesions should suggest other etiologies (PML, lymphoma). Lesions often occur in the basal ganglia and corticomedullary junctions.
• Examination: Focal deficits proportional to lesion location; seizures in ~30% of cases; altered consciousness if edema is severe.

Cytomegalovirus disease

• CMV retinitis: Progressive visual field loss, "floaters," photopsia. Typically painless unless uveitis develops. Classic descriptions include "cottage cheese and ketchup" hemorrhagic retinopathy or "granular" appearance with minimal hemorrhage. Often asymptomatic initially until central (macula) involvement occurs.
• CMV colitis: Abdominal pain, diarrhea (often bloody), weight loss. May progress to colonic perforation or toxic megacolon.
• CMV esophagitis: Dysphagia, odynophagia, substernal chest pain. Ulcers are typically large and shallow (vs. HSV ulcers which are smaller and numerous).
• CMV ventriculoencephalitis or polyradiculopathy: Presents with progressive lower extremity weakness, sensory loss, and urinary retention (polyradiculopathy); or confusion and altered mentation (ventriculoencephalitis).
• Examination: Visual field defects on confrontation testing; perirectal tenderness on abdominal exam in colitis; lower extremity weakness and hyporeflexia in polyradiculopathy.

Cryptosporidiosis

• Chronic watery diarrhea: Profuse, watery diarrhea (up to 10-20 L/day) with malabsorption, weight loss, and electrolyte derangements. Diarrhea is often noroviral in onset and may be intermittent. Abdominal cramping occurs but is usually mild.
• Cholangitis/biliary disease: Bile duct strictures and sclerosing cholangitis can develop, presenting with right upper quadrant pain, fever, and elevated alkaline phosphatase/GGT.
• Examination: Dehydration, wasting, abdominal distention may be present; fever is variable.

Candidiasis

• Oral thrush: White pseudomembranes on tongue, hard palate, buccal mucosa; erythema; dysgeusia; difficulty eating. Candida grows as commensal fungus; loss of local immunity permits overgrowth.
• Esophageal candidiasis: Dysphagia, odynophagia (substrate for esophageal erosions/ulceration). May present with chest pain mimicking cardiac ischemia if severe. Candida in esophagus almost always indicates CD4 <100 and warrants empiric ART consideration.
• Vaginal candidiasis: Vulvar erythema, cottage-cheese discharge, pruritus (may be recurrent and more severe with lower CD4).
• Examination: Characteristic white pseudomembranes in oral cavity; erythema of oropharynx.

Mycobacterium avium complex

• Fever, night sweats, abdominal pain, and chronic diarrhea: MAC bacteremia develops insidiously with constitutional symptoms, elevated inflammatory markers (ESR, CRP), and often hepatosplenomegaly. Some patients are asymptomatic at diagnosis.
• Lymphadenitis (often localized vs. disseminated): MAC can present as localized lymph node disease or disseminated bacteremia. CD4 almost always <50.
• Examination: Hepatosplenomegaly, abdominal lymphadenopathy (may be palpable), wasting. Lymph nodes may be necrotic and suppurative (MAC lymphadenitis after immune reconstitution).

Cytomegalovirus and other herpesviruses

• Herpes simplex virus (HSV): Severe

Pneumocystis pneumonia

• Initial tests: chest radiograph showing diffuse bilateral perihilar interstitial or ground-glass infiltrates, elevated LDH, and elevated serum (1,3)-β-D-glucan. Exercise oximetry showing desaturation supports the diagnosis. A normal CXR does not exclude PCP; HRCT shows ground-glass opacity.
• Confirmatory test: microscopic identification of cysts in induced sputum or bronchoalveolar lavage using methenamine silver, Giemsa, or direct fluorescent antibody stain; PCR is highly sensitive but cannot distinguish colonization. Pneumocystis cannot be cultured.
• Severity grading: arterial blood gas on room air — a PaO₂ below 70 mmHg or an alveolar–arterial gradient of 35 mmHg or greater defines moderate-to-severe disease and triggers adjunctive steroids.

CNS disease

• Toxoplasmic encephalitis: contrast MRI showing multiple ring-enhancing lesions with predilection for basal ganglia and gray–white junction, plus positive Toxoplasma IgG. Diagnosis is presumptive and confirmed by clinical and radiographic response to empiric therapy within about two weeks; stereotactic brain biopsy is the gold standard reserved for non-responders.
• Discriminators: CNS lymphoma is typically solitary and periventricular with EBV DNA in CSF; PML shows non-enhancing white-matter lesions without mass effect and JC virus PCR in CSF.
• Cryptococcal meningitis: serum or CSF CrAg lateral flow assay is highly sensitive and, with fungal culture, is the diagnostic mainstay; confirm with lumbar puncture measuring opening pressure. India ink preparation is only supportive — it is substantially less sensitive than CrAg, so a negative India ink does not exclude disease. CSF typically shows scant lymphocytic pleocytosis with markedly elevated opening pressure.

Other organ-specific confirmation

• Disseminated MAC: mycobacterial blood cultures.
• Tuberculosis: sputum AFB smear plus nucleic acid amplification (Xpert MTB/RIF) with mycobacterial culture and drug-susceptibility testing.
• CMV retinitis: diagnosis is clinical by dilated indirect ophthalmoscopy; serum CMV PCR is neither sensitive nor specific for retinitis.
• Cryptosporidiosis: modified acid-fast stain or stool antigen/PCR.
• Esophageal candidiasis: treat empirically; endoscopy with brushings only if no response.

Immediate stabilization

• Hypoxemic respiratory failure (PCP): supplemental oxygen, and if PaO₂ is below 70 mmHg or the A–a gradient is 35 mmHg or greater, give adjunctive corticosteroids (prednisone, tapered over 21 days) before or with the first antimicrobial dose — steroids blunt the inflammatory surge from organism lysis and reduce mortality and intubation.
• Raised intracranial pressure (cryptococcal meningitis): therapeutic lumbar puncture for opening pressure ≥25 cm H₂O, repeated daily as needed. Mannitol, acetazolamide, and corticosteroids are not effective and adjunctive dexamethasone worsens outcomes.

First-line therapy by organism

• PCP: folate-pathway inhibitor combination — TMP-SMX, IV or oral, for 21 days.
• Toxoplasmic encephalitis: pyrimethamine plus sulfadiazine plus leucovorin; TMP-SMX is an accepted alternative.
• Cryptococcal meningitis: induction with liposomal amphotericin B plus flucytosine, then consolidation with high-dose fluconazole, then lower-dose fluconazole maintenance until immune reconstitution.
• Disseminated MAC: macrolide (azithromycin or clarithromycin) plus ethambutol, with rifabutin added for severe disease or high mycobacterial burden.
• CMV retinitis: oral valganciclovir, with intravitreal ganciclovir or foscarnet added for immediately sight-threatening lesions near the macula or optic nerve.
• TB: standard RIPE therapy, substituting rifabutin for rifampin when a protease inhibitor is used, because rifampin's potent CYP3A4 induction collapses PI levels.

Escalation and alternatives

• PCP sulfa intolerance: clindamycin plus primaquine, IV pentamidine, or atovaquone for mild disease. Check G6PD before primaquine or dapsone.
• Toxoplasmosis non-response at ~2 weeks: pursue brain biopsy rather than escalating empirically.
• Cryptosporidiosis: no reliably curative agent — ART-driven immune reconstitution is the definitive therapy; nitazoxanide is adjunctive.

ART timing and contraindications

• Start ART within about two weeks of beginning OI treatment for most infections, including PCP and MAC.
• Defer ART in CNS infections: several weeks in cryptococcal meningitis (early ART increased mortality in the COAT trial), and approximately 8 weeks in TB meningitis regardless of CD4 count — in contrast to non-meningeal TB, where ART begins within about two weeks if CD4 is very low.
• Contraindicated/avoid: corticosteroids in cryptococcal meningitis; pyrimethamine without leucovorin; rifampin with protease inhibitors.

Disease-related — emergencies flagged

• Respiratory failure in PCP (emergency): alveolar filling with foamy exudate plus the inflammatory response to organism lysis produces worsening hypoxemia in the first days of therapy; signaled by rising oxygen requirement and falling PaO₂. This is the rationale for pre-emptive steroids.
• Pneumothorax in PCP (emergency): cystic/pneumatocele destruction of lung parenchyma; signaled by sudden pleuritic pain and unilateral decreased breath sounds.
• Cerebral herniation from raised ICP in cryptococcal meningitis (emergency): arachnoid granulation obstruction by polysaccharide capsule impairs CSF resorption; signaled by depressed consciousness, new cranial nerve VI palsy, or visual loss.
• Mass effect and status epilepticus in toxoplasmic encephalitis (emergency): perilesional vasogenic edema; signaled by progressive focal deficit or seizures.
• Retinal detachment and irreversible blindness in CMV retinitis: full-thickness necrotic retina thins and tears; signaled by acute visual field loss.
• Colonic perforation or toxic megacolon in CMV colitis (emergency): transmural vasculitic ulceration; signaled by peritonitis and free air.
• Sclerosing cholangitis in cryptosporidiosis: biliary epithelial infection producing strictures; signaled by RUQ pain with disproportionate alkaline phosphatase elevation.

Treatment-related

• TMP-SMX: rash progressing to Stevens–Johnson syndrome, myelosuppression, hyperkalemia (epithelial sodium channel blockade), and a benign creatinine rise from inhibited tubular creatinine secretion without true GFR loss.
• Pyrimethamine and flucytosine: bone marrow suppression from folate antagonism and antimetabolite effect; leucovorin is mandatory with pyrimethamine.
• Amphotericin B: nephrotoxicity with potassium and magnesium wasting plus infusion reactions.
• Ganciclovir/valganciclovir: neutropenia. Foscarnet: nephrotoxicity and hypocalcemia/hypomagnesemia causing seizures and tetany. Cidofovir: proximal tubular injury.
• Ethambutol: optic neuritis with red-green dyschromatopsia. Rifabutin: anterior uveitis and neutropenia, worsened when PIs raise its level.
• Pentamidine: hypoglycemia then hyperglycemia from pancreatic islet toxicity; QT prolongation.

• CD4 thresholds are the stem's hidden question: below 200 think PCP; below 100 think Toxoplasma and cryptococcus; below 50 think MAC and CMV retinitis. Match the number to the organism before reading the imaging.
• Hypoxemia out of proportion to the chest film, elevated LDH, and an HIV patient not on prophylaxis is PCP until proven otherwise. The single best next step when PaO₂ is below 70 mmHg or the A–a gradient is 35 mmHg or greater is corticosteroids given with (not after) TMP-SMX — the CDC/NIH/IDSA OI guidelines make this explicit.
• **Multiple ring-enhancing lesions + positive Toxoplasma IgG → treat empirically with pyrimethamine–sulfadiazine–leucovorin and reimage in two weeks; biopsy only if no response. The distractor is jumping to brain biopsy first. A solitary periventricular lesion with EBV DNA in CSF** is primary CNS lymphoma, not toxoplasmosis.
• Cryptococcal meningitis is a pressure disease as much as an infection: serial therapeutic lumbar punctures for opening pressure ≥25 cm H₂O. Steroids are harmful here — a favorite trap given how reflexively dexamethasone is chosen for bacterial meningitis. Remember also that a negative India ink does not rule out disease; CrAg and fungal culture do the diagnostic work.
• Defer ART in cryptococcal and TB meningitis; start it early in most other OIs. Examiners test the paradox that giving ART too soon in CNS cryptococcosis increases mortality through IRIS.
• Esophagitis lesion morphology: large, solitary, shallow, linear/serpiginous ulcers = CMV; small, multiple, punched-out (volcano-like) ulcers = HSV; white plaques = Candida. Odynophagia in an HIV patient gets empiric fluconazole first, endoscopy only if it fails.
• Rifampin plus a protease inhibitor is the wrong pairing — substitute rifabutin, because rifampin's CYP3A4 induction destroys PI levels. Conversely, watch for rifabutin-induced uveitis.
• Check G6PD before dapsone or primaquine, and remember TMP-SMX raises creatinine by blocking tubular secretion without reducing true GFR — do not misread this as acute kidney injury and stop the drug.