Gut microbiota sustains hematopoiesis. Compounds of the commensal gut microbiota stimulate lymphocytes, macrophages, and dendritic cells in the lamina propria, which, in concert, produce a series of extrinsic stimuli. Together, these microbial and cellular stimuli maintain tonic activity of hematopoietic stem cells (HSC) and hematopoietic progenitor cells (HPC) as well as lymphocytes, monocytes, and neutrophils. Hence, microbial compounds promote steady-state hematopoiesis and vigilance of the innate and adaptive immune system against bacterial and viral infections. Broad-spectrum antibiotic treatment (ABX) can disrupt the balance and diversity of commensal gut microbiota substantially, leading to impaired hematopoiesis and a higher susceptibility to infections. RBC, red blood cell. Professional illustration by Patrick Lane, ScEYEnce Studios.