Mothers and their babies represent one of the closest dyadic units and thus provide a powerful paradigm to examine how affective states are shared, and result in, synchronized physiologic responses between two people. We recruited mothers and their 12- to 14-month-old infants (Ndyads = 98) to complete a lab study in which mothers were initially separated from their infants and assigned to either a low-arousal positive/relaxation condition, intended to elicit parasympathetic nervous system (PNS) reactivity, or a high-arousal negative/stress task, intended to elicit sympathetic nervous system (SNS) reactivity. Upon reunion, infants were placed either on their mothers’ laps (touch condition) or in a high chair next to the mother (no-touch condition). We then examined if the babies SNS and/or PNS responses changed from their baseline levels and how the dyads’ physiological responses—both PNS and SNS responses—synchronized over time as a function of mothers’ affect manipulation and touch condition. Three noteworthy findings were observed. First, infants of mothers assigned to the relaxation task showed greater PNS increases and PNS covariation. Second, infants of mothers assigned to the stress task showed stronger SNS covariation with their mothers over time. Finally, infants who sat on their mothers’ laps (i.e., touch condition) showed stronger SNS covariation than those in the no-touch condition. Taken together, these results suggest that mothers’ affective states—low-arousal positive states as well as high-arousal negative states—can be “caught” by their infants, and that touch can play a critical role in stress contagion.