Clinical deterioration during the course of community-acquired infections can occur as a result of an exaggerated immune response of the host towards the inciting pathogens, leading to immune-mediated tissue damage. Whether a surge in the peripheral lymphocyte count can be used as a surrogate marker indicating the onset of immunopathological tissue damage is not known. In this study, we report the clinical presentations and outcomes of a cohort of immunocompetent patients with non-tuberculous community acquired infections who experienced clinical deterioration during hospital stay (n=85). 12 (14.1%) patients had a surge in lymphocyte count preceding their clinical deteriorations, and their diagnoses included viral pneumonitis , viral encephalitis , scrub typhus , leptospirosis , brucellosis , and dengue haemorrhagic fever . The clinical manifestations during deterioration ranged from interstitial pneumonitis , airway obstruction , CNS disturbances , and systemic capillary leak syndrome , all of which were thought to represent immunopathological tissue damages. When compared with patients without lymphocyte surge, these patients were more likely to be infected with fastidious/viral pathogens (0 vs 12; p<0.05), in addition to having lower mean baseline lymphocyte counts (403+/-181 vs 1143+/-686 cells/microl; p<0.05). We postulate that the peripheral lymphocyte count may be a useful surrogate marker indicating the presence of immunopathological damage during clinical deterioration in certain infectious diseases.