Latently-infected aged mice are protected against re-infection by γHV68. Young mice (3 months pi; circles) and aged mice (19 months pi; squares) were intranasally challenged with γHV68 (400 PFU, closed symbols or 3000 PFU, open symbols). Naïve mice were challenged as a control (triangles). Symbols indicate the titers of lytic γHV68 virus detected in the lungs of individual mice at 3 and 6 dpi. The dotted line represents the limit of detection for the plaque assay used to measure viral titers. Chi-square analysis comparing numbers of mice with or without plaques detectable in lungs revealed significant differences (P < 0.05) only between the naïve mice challenged as a control and the other four groups.