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Objective: Schizophrenia and bipolar disorder are associated with cognitive impairment leading to social disruption. While previous studies have focused on the effect of individual infectious exposure, namely, Herpesviridae viruses or Toxoplasma gondii (T gondii), on cognitive functioning, the objective of the present study was to examine the effect of multiple infections on cognitive functioning in patients with schizophrenia and bipolar disorder and in healthy controls. Methods: Seropositivity to herpes simplex virus type 1 (HSV-1), herpes simplex virus type 2 (HSV-2), cytomegalovirus (CMV), and T gondii was related to cognitive status among 423 participants (recruited between 2008 and 2014; 138 patients with bipolar disorder, 105 patients with schizophrenia [DSM-IV criteria], and 180 healthy controls) for episodic verbal memory (California Verbal Learning Test), working memory (Wechsler Adult Intelligence Scale, third edition), and premorbid intelligence quotient (National Adult Reading Test). Results: Seropositivity to and antibody levels of HSV-1 were significantly associated with working memory, which persisted after correction (backward digit span: beta = -0.10 [0.05], chi(2) = 33.89, P = .0001) in the overall sample. This association was particularly strong in the control group (beta = -0.18 [0.08], P = .04, Z = -3.55, P = .0008; corrected P = .012). Further, cumulative exposure to HSV-1, HSV-2, and CMV viruses and T gondii parasite was also associated with lower scores on working memory as measured by backward digit span in the overall sample (Z = 2.86, P = .004; Z = 2.47, P = .01; and Z = 3.35, P = .01, respectively). Conclusions: Exposures to Herpesviridae and T gondii parasite seem to impact cognitive functioning. Because infections caused by Herpesviridae and/or T gondii parasite are quite common in the (general) population, assessing and confirming the cognitive impairment among those who have cumulative exposures is useful and of interest.