The microbes inhabiting our bodies have a profound influence on our physical health, but they are also highly involved in our mental health. An obvious influence is that bacteria can often make humans ill, which then can cause health anxiety, and in some cases, depression. However, there is also evidence that microbes can alter personality in mice. Mice kept in germ-free environments tended to be more hyperactive and daring than mice with more developed microbiota. Additionally, there has been a correlation between autism development and altered microbiomes. The behaviors of a certain subsection of mice very much mimicked those seen in autistic humans, and the imbalances seen in their gut microbiomes could potentially be a cause of their behavioral symptoms. Furthermore, it appears that gut microbiota communicate with the brain via the vagus nerve, which ultimately alters the permeability of the blood-brain barrier.