The two extremes – smile and stress are contagious. However, stress also alters the brain on a cellular level.
According to a study published in the journal Nature Neuroscience, stress transmitted from others can alter the brain in the same way as real stress does. The study also shows that the reversible effects are different in both genders. 
Researchers from the University of Calgary performed the study on mice to find out the impact of the contagiousness of stress. “What we can begin to think about is whether other people’s experiences or stresses may be changing us in a way that we don’t fully understand,” said Dr. Jaideep Bains, professor at the Cumming School of Medicine. 
The researchers performed the study on pairs of male and female mice. One mouse from each pair was exposed to brief electroshocks on their feet and then allowed to return to their partners. They then studied the CRH neurons in the brain, which are responsible for brain’s response to stress. This showed that the networks of both the partners in both groups were altered in the same way. The reason is that the activation of the CRH neuron released a chemical signal ‘alarm pheromone’, which was responsible for the brain altering. Anyone who detects this signal can easily spread the tension to others in the group.
On the contrary, social groups are also responsible to buffer these effects, however, this is gender biased. When the female stressed partners spent time with unstressed partners, over time, the CRH neurons were cut to almost half. This was not the same for the males.
Extending the findings to humans, Bain concluded, “We readily communicate our stress to others, sometimes without even knowing it. There is even evidence that some symptoms of stress can persist in family and loved ones of individuals who suffer from PTSD. On the flip side, the ability to sense another’s emotional state is a key part of creating and building social bonds.”