Purpose The purpose of this study was to test the hypothesis that exposure to a directly transmitted human pathogen—flu virus—increases human social behavior presymptomatically. This hypothesis is grounded in empirical evidence that animals infected with pathogens rarely behave like uninfected animals, and in evolutionary theory as applied to infectious disease. Such behavioral changes have the potential to increase parasite transmission and/or host solicitation of care.

Methods We carried out a prospective, longitudinal study that followed participants across a known point-source exposure to a form of influenza virus (immunizations), and compared social behavior before and after exposure using each participant as his/her own control.

Results Human social behavior does, indeed, change with exposure. Compared to the 48 hours pre-exposure, participants interacted with significantly more people, and in significantly larger groups, during the 48 hours immediately post-exposure.