HIV

Also known as: Human immunodeficiency virus, AIDS

HIV (human immunodeficiency virus) is a retrovirus that infects and progressively depletes CD4 positive T lymphocytes, causing progressive immunodeficiency and, without treatment, susceptibility to a characteristic range of opportunistic infections and malignancies collectively defining acquired immunodeficiency syndrome (AIDS).

Transmission occurs through sexual contact, blood and blood product exposure (including shared injecting equipment), and vertical (mother to child) transmission during pregnancy, delivery, or breastfeeding.

With effective antiretroviral therapy, HIV has been transformed from a uniformly fatal condition into a manageable chronic disease, with near normal life expectancy achievable in patients diagnosed and treated promptly and who maintain good adherence, which underscores the importance of both early diagnosis and sustained engagement in care.

Acute HIV infection (seroconversion illness), occurring typically 2 to 4 weeks after exposure, presents with a non-specific, often self limiting mononucleosis like illness: fever, lymphadenopathy, pharyngitis, myalgia, and a maculopapular rash, frequently unrecognized or misattributed to a viral illness at the time.
A prolonged clinically latent period, often lasting many years, follows, during which the patient may be entirely asymptomatic despite ongoing viral replication and gradual CD4 count decline.
As CD4 count falls, patients become progressively susceptible to a range of characteristic opportunistic infections and conditions, generally occurring at recognized, roughly predictable CD4 thresholds: oral or esophageal candidiasis and reactivation of latent tuberculosis at moderately reduced CD4 counts; Pneumocystis jirovecii pneumonia, cerebral toxoplasmosis, and cryptococcal meningitis at more severely reduced counts (typically below 200 cells per microliter); and cytomegalovirus retinitis, disseminated Mycobacterium avium complex infection, and progressive multifocal leukoencephalopathy at the most severely reduced counts (typically below 50 cells per microliter).
AIDS defining malignancies include Kaposi sarcoma (associated with human herpesvirus 8 co-infection, presenting with characteristic violaceous skin, mucosal, or visceral lesions), non-Hodgkin lymphoma, and, in women, invasive cervical cancer, reflecting the broader increased malignancy risk associated with chronic immunodeficiency and, for some of these, co-infection with oncogenic viruses.

Fourth generation combination antigen and antibody testing is the standard initial screening test in most settings, detecting both HIV antibody and p24 antigen, which narrows the diagnostic window period compared to older antibody only assays, since p24 antigen becomes detectable before antibody seroconversion is complete.
A reactive screening test is confirmed with a supplemental, differentiation assay distinguishing HIV-1 from HIV-2, per standard testing algorithms.
HIV RNA viral load testing can detect infection during the window period before antibody seroconversion is complete, and is particularly useful when acute HIV infection is clinically suspected despite a negative or indeterminate antibody based test.
Once diagnosis is confirmed, baseline assessment includes CD4 count (establishing the degree of immunodeficiency and guiding opportunistic infection prophylaxis needs), HIV viral load (establishing a baseline before treatment), genotypic resistance testing (guiding initial antiretroviral regimen selection), and screening for common coinfections, including hepatitis B, hepatitis C, syphilis, and tuberculosis, alongside other routine baseline bloods.

Differentials for the acute seroconversion illness include infectious mononucleosis and other viral illnesses. The differential for specific opportunistic infections and malignancies is addressed within their own respective entries.

Antiretroviral therapy is now recommended for all patients diagnosed with HIV, regardless of CD4 count, reflecting a shift from older, CD4 threshold based initiation criteria, given clear evidence that early treatment improves individual outcomes and reduces onward transmission risk.
Modern first line regimens typically combine two nucleoside or nucleotide reverse transcriptase inhibitors with a third agent, most commonly an integrase strand transfer inhibitor (such as dolutegravir or bictegravir) given their high efficacy, good tolerability, and high barrier to resistance, often available as convenient single tablet, once daily fixed dose combinations that support adherence.
Adherence is the single most important determinant of treatment success, since inconsistent adherence risks the development of resistance and subsequent treatment failure; adherence support, addressing practical, psychological, and social barriers, is a core and ongoing component of care rather than a one time counseling point.
Opportunistic infection prophylaxis is indicated at specific CD4 thresholds: co-trimoxazole prophylaxis against Pneumocystis jirovecii pneumonia and toxoplasmosis is typically started when CD4 falls below approximately 200 cells per microliter, and can be discontinued once the CD4 count has recovered above this threshold on effective antiretroviral therapy for a sustained period.
Immune reconstitution inflammatory syndrome is a recognized complication of starting antiretroviral therapy, particularly in patients with a very low CD4 count and an underlying, sometimes previously subclinical, opportunistic infection.
Regular monitoring includes CD4 count and viral load at intervals guided by treatment stability (viral load should become undetectable within several months of starting effective therapy, and maintaining an undetectable viral load is both the marker of treatment success and, importantly, means the person cannot sexually transmit HIV, the basis of the "undetectable equals untransmittable" principle now central to modern HIV counseling).
Pre-exposure prophylaxis (PrEP), using antiretroviral medication in HIV negative individuals at ongoing substantial risk of acquisition, and post-exposure prophylaxis (PEP), started as soon as possible (and within 72 hours) after a specific high risk exposure, are both important prevention tools, complementing treatment as prevention in reducing population level HIV transmission.
Prevention of mother to child transmission combines maternal antiretroviral therapy throughout pregnancy (ideally with a fully suppressed viral load well before delivery), consideration of mode of delivery based on viral load near term, neonatal post exposure prophylaxis, and guidance on infant feeding, together reducing transmission risk to well under 1% in optimally managed settings.

Referral: HIV or infectious disease specialist service for all newly diagnosed patients, for treatment initiation and ongoing long term management; obstetric and specialist HIV input for all pregnant patients living with HIV, given the specific considerations around preventing vertical transmission.

Clinical toolsCalculators

BMI GCS EDD Parkland Formula

Corrected Sodium Corrected Calcium Anion Gap Creatinine Clearance Insulin Calculations qSOFA Wells PE Wells DVT CHA₂DS₂-VASc HAS-BLED CURB-65 Centor/McIsaac

Infectious Myositis

Infectious myositis refers to infection of skeletal muscle, ranging from localized pyomyositis (a discrete bacterial abscess within a muscle, most commonly caused by Staphylococcus aureus) to more diffuse or systemic infectious causes of muscle inflammation (viral myositis, and, in the tropics, various parasitic causes including trichinellosis and, rarely, cysticercosis, see that entry).

Pyomyositis was historically considered a tropical disease (also termed "tropical pyomyositis"), but is increasingly recognized in temperate settings, particularly in the context of diabetes, immunosuppression, IV drug use, or preceding trauma or muscle strain that may seed hematogenous bacteria into an area of localized muscle injury.

Pyomyositis: presents in three overlapping stages, an early invasive stage with localized, dull, cramping muscle pain, mild fever, and woody induration without fluctuance (often misdiagnosed initially as a muscle strain or hematoma given the absence of overt abscess features), followed by a suppurative stage (days to weeks later) with a more clearly localized, tender, sometimes fluctuant mass and systemic toxicity, and, if untreated, a late stage with systemic sepsis and risk of metastatic infection elsewhere. Most commonly affects the large muscles of the thigh, calf, or gluteal region
Viral myositis (commonly influenza, particularly in children, and less commonly other respiratory viruses or coxsackievirus) presents with acute, often bilateral, symmetric muscle pain and tenderness, classically affecting the calves, alongside the systemic features of the underlying viral illness, and is typically self-limiting
Both can cause elevated creatine kinase, and severe cases (of either etiology) carry a risk of rhabdomyolysis with resulting acute kidney injury

MRI is the imaging investigation of choice for suspected pyomyositis, showing the characteristic muscle edema and, where a discrete abscess has formed, a rim-enhancing fluid collection, with substantially higher sensitivity than ultrasound or CT for early-stage disease
Blood cultures should be sent given the frequency of accompanying bacteremia, alongside FBC and inflammatory markers (typically significantly elevated)
CK should be checked in any suspected myositis, both to support the diagnosis and to screen for rhabdomyolysis
Image-guided or surgical aspiration/drainage of any identified collection provides both source control and definitive microbiological diagnosis via Gram stain and culture
Viral myositis diagnosis is largely clinical in the context of a compatible viral prodrome, with specific viral testing (e.g. influenza PCR) supporting the diagnosis where relevant to management decisions

Pyomyositis: drainage of any discrete abscess (image-guided percutaneous or open surgical) is central to management, particularly once frank pus has formed, combined with empirical IV antibiotics covering Staphylococcus aureus (including MRSA cover where locally prevalent or risk factors are present): IV flucloxacillin 2 g qds, or, where MRSA is a concern, IV vancomycin (dosed and monitored per local protocol) or clindamycin, adjusted once culture results are available; total duration typically 3 to 4 weeks, guided by clinical and radiological response, with an initial IV course transitioning to oral therapy as improvement is established
Early-stage disease without a discrete drainable collection may respond to antibiotics alone with close monitoring, but a low threshold for repeat imaging and drainage should be maintained if response is inadequate
Viral myositis: supportive management (rest, analgesia, hydration), with monitoring of CK and renal function given the rhabdomyolysis risk, particularly in more severe cases; specific antiviral treatment (e.g. oseltamivir for influenza, see Influenza entry) may be indicated based on the underlying viral cause and its own treatment criteria
Rhabdomyolysis, if present, is managed with aggressive IV fluid resuscitation and monitoring of renal function and electrolytes (see Acute Kidney Injury entry)

Referral: interventional radiology or surgery for drainage of pyomyositis abscess; infectious disease for antibiotic selection and duration guidance, particularly in atypical or refractory cases.

HIV

Overview

Clinical Features

Diagnosis

Management

Overview

Clinical Features

Diagnosis

Management