A satellite view of Baltimore, Maryland, would show plenty of abandoned buildings and parking lots, with parks—such as Patterson and Gwynns Falls parks—scattered throughout. However, while there is an abundance of concrete and asphalt within the city limits, Baltimore is not a city in isolation. Like Washington, D.C. and other nearby urban areas, Baltimore lies within the Chesapeake Bay watershed.
Scientists have known for decades that the human intestinal tract is home to an abundance of diverse bacteria. This microbial rainforest is introduced incrementally to infants as they grow—primarily from their mothers during birth and breastfeeding and from everyday encounters. Many of these microbes aid in digestion and fight off pathogens, but until recently, researchers were not certain if phages, viruses that infect bacteria, were also present in the human gut.