Doctors who prescribe antibiotics to children indiscriminately aren’t doing their patients any favors.

There are many good reasons to avoid doling out these medications carelessly to patients of all ages. However, a paper in the journal Cell Host & Microbe offers insight into why pediatric patients may be particularly vulnerable to ill effects from overuse of antibiotics—such use may increase their risk of several diseases later in life, including obesity.

From the first moments of an infant’s life, its gut is already being populated by bacteria from the mother’s body. These bacteria will go on the play an important role in regulating the long-term health of the infant, not just in the newborn period, but potentially for years to come. As the paper describes in detail, this process can be significantly affected by numerous factors, including whether the baby was born vaginally or by Cesarean section, or whether it is breastfed or given formula. Even the mother’s weight can affect the diversity of bacteria that populate the child’s gastrointestinal tract.