The two groups of people with AHD – those at high risk and those who are seriously ill – may overlap and individuals can move between groups: people who are asymptomatic but with a very low CD4 cell count can rapidly become seriously ill, and after initial clinical management people recovering from serious illness will benefit from ongoing prophylaxis, differentiated service delivery and holistic supportive community care, perhaps including home visits. The gene discussed is CD4; the disease is mitochondrial DNA depletion syndrome 4a.