Primary HIV infection is a critical period in which the virological and immunological events that take place, even before the onset of clinical symptoms, may determine the long-term outcome of disease. Early recognition of symptoms, especially in people deemed to be high-risk individuals, is critical. Understanding the pathophysiology of PHI allows appropriate selection of diagnostic tests in the acute period. Early diagnosis allows prompt institution of ART, which has highly significant health benefits in the individual, and is fundamental to the concept of treatment as prevention of further transmission, conferring significant public health benefit.