Microorganisms in the human gut can affect immune-system cells. Gut bacterial strains have been discovered that boost immune cells that have cell-killing capacity and that can target cancer and protect against infection.

Reference: Nathan E. Reticker-Flynn , Edgar G. Engleman, 23 January 2019, A gut punch fights cancer and infection, https://www.nature.com/articles/d41586-019-00133-w?WT.ec_id=NATURE-201901&sap-outbound-id=9CB8419C62277C0A4ECDF43F8F445EC44929733D&fbclid=IwAR1UcFyaRw4_qGp2tSlpSGfUIV7ijhwY_HtOyfeXBoGVRelut0QLlqds_FQ