Acute Epstein-Barr virus (EBV) infection results in an unusually robust CD8(+) T cell response in young adults. Based on mouse studies, such a response would be predicted to result in attrition of preexisting memory to heterologous infections like influenza A (Flu) and cytomegalovirus (CMV). Furthermore, many studies have attempted to define the lymphocytosis that occurs during acute EBV infection in humans, but it is unclear whether bystander T cells contribute to it. To address these issues, we performed a longitudinal prospective study of primary EBV infection in humans. During acute EBV infection, both preexisting CMV- and Flu-specific memory CD8(+) T cells showed signs of bystander activation, including up-regulation of granzyme B. However, they generally did not expand, suggesting that the profound CD8(+) lymphocytosis associated with acute EBV infection is composed largely of EBV-specific T cells. Importantly, the numbers of CMV- and Flu-specific T cells were comparable before and after acute EBV infection. The data support the concept that, in humans, a robust CD8(+) T cell response creates a new memory CD8(+) T cell niche without substantially depleting preexisting memory for heterologous infections