It has long been known among medical professionals that taking broad-spectrum antibiotics over a long period of time can lead to severe secondary bacterial infections and the overall weakening of the immune system. Only now researchers from the University of Pennsylvania School of Medicine seem to have figured out why this is so. “One of the complications of antibiotic therapy is secondary infection,” Jeffrey Weiser, MD, professor of Microbiology and Pediatrics explains. “This is a huge problem in hospitals, but there hasn’t been a mechanistic understanding of how that occurs. We suggest that if the immune system is on idle, and you treat someone with broad-spectrum antibiotics, then you turn the system off. The system is deprimed and will be less efficient at responding quickly to new infections.” For many years researchers have understood that most bacteria in the body are good. In fact, humans have a symbiotic relationship with these microbes that greatly impact, among other things, metabolism and weight homeostasis. Now we have an even more clear view of bacterial microbes’ effect on the immune system.