Healthier groups may also be more peaceful groups
Research into group and individual behavior suggests that the rate of infectious disease in a group is significantly associated with the rate of violent and property crime and with the rate of violent crime against strangers. 
"Disease threat activates responses that function to guard against the threat, such as out-group avoidance and in-group preference. When these responses are widespread among many individuals—for example, in places where infection risk is chronically elevated—they foster xenophobia ["fear or hatred of strangers or foreigners," 2] and produce a fragmented social structure that increases the potential for out-group aggression (Letendre et al., 2010). We found that infection rates explained a substantial proportion of the variance in violent and property crimes, and in most cases was a stronger predictor than established crime covariates. Interestingly, the homicide results showed that the aggression was directed primarily at out-group members, with pathogens predicting stranger homicides more strongly than any of the control variables."
"Only diseases with over 20,000 new cases in at least one of the five years examined (1995–1999) were used. This criterion left us with eight diseases: AIDS, Chlamydia, gonorrhea, syphilis, Hepatitis A, salmonellosis, shigellosis, and tuberculosis. These eight diseases accounted for over 90% of all the infection counts reported in this system over these years; the incidence of the other diseases (e.g., cholera, malaria) tended to be very low."
- I. Shrira, A. Wisman, G. D. Webster, [1, "Guns, Germs, and Stealing: Exploring the Link between Infectious Disease and Crime," 2013]
Historically a condition known as amok was first described in 1893 where an individual suddenly acted violently and then would forget the manic episode. Descriptions of the disorder were recorded by the British medical superintendent of the Government Asylum in Singapore. Increases in cases of amok were more associated with "times of social tension or impending disaster." The use of the term has become more commonly associated in modern times with the phrase "running amok" which is not a medical term but more of a slang phrase. [Infectious Madness, by Harriet A. Washington, page 163, 3] Gun violence in modern times has involved racist xenophobia in many cases and in a few cases may have involved a shooter who claimed to have amnesia of the event afterwards. [4, 5]
To have healthier groups we may need less stress overall and more jobs and stability in our local and global economies. Working together to help achieve healthier groups may require us first to recognize our subtle tendencies to distrust others during times of increased rates of illness.
/Disclaimer: This information is provided for educational purposes within the guidelines of fair use. Information is not a substitute for individual health guidance. Please see a health professional for individual health care purposes./