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Background: Recently, a negative association between Toxoplasma -infection and novelty s eeking was reported. The authors suggested that changes of pers onality trait were caused by manipula tion activity of the parasite, aimed at increasing the probability of tr ansmission of the parasite from an intermedia te to a definitive host . They also suggested that low novelty seeking indicated an in creased level of the neurot ransmitter dopamine in the brain of infected subjects, a phenomenon already observ ed in experimentally infected ro dents. However, the changes in personality can also be just a byproduct of any neurotropic infection . Moreover, the association between a pe rsonality trait and the toxoplasmosis can even be caused by an independent correlation of both the probability of Toxoplasma -infection and the personality trait with the third factor, namely with th e size of living place of a subject. To test these two alternative hypotheses, we studied the influence of another neurotropic pathogen, the cyto megalovirus, on the personali ty of infected subjects, and reanalyzed the original data after the effect of the potential confounder, the si ze of living plac e, was controlled. Methods: In the case-control study, 533 conscr ipts were tested for to xoplasmosis and presence of anti-cytomegalovirus antibodies and their novelty se eking was examined with Cloninger's TCI questionnaire. Possible association between the two infections and TCI dimensions was analyzed. Results: The decrease of novelty seeking is as sociated also with cytomegalovirus infe ction. After the size of living place was controlled, the effect of to xoplasmosis on novelty seeking increased. Si gnificant difference in novelty seeking was observed only in the largest city, Prague. Conclusion: Toxoplasma and cytomegalovirus probably induce a decrease of novelty seeking. As the cytomegalovirus spreads in population by direct contact (not by predation as with Toxoplasma ), the observed changes are the byproduct of brain infections rather than the result of manipulation activity of a parasite . Four independent line s of indirect evidence, namely direct measurement of neurotransmi tter concentration in mice, the nature of behavioral changes in rodents, the nature of personality changes in humans, and the observed association between schizophren ia and toxoplasmosis, suggest that the changes of dopamine concentration in brain coul d play a role in behavioral changes of infected hosts.