Antiretroviral therapy (ART) is usually effective at suppressing HIV, allowing the immune system to recover by preventing the virus from destroying CD4+ T cells. Scientists have now identified a rare, paradoxical response to ART called extreme immune decline, or EXID. Five individuals evaluated at the NIAID experienced a significant decline in CD4+ T cell levels despite suppression of HIV below detectable levels for more than three years, according to a new report.