Soluble fibre deficiency & unhealthy gut microbiota.
Soluble fibre is essential, and instrumental in a healthy dietary/immune system. The center of the dietary immune system is your gut microbiota.
Prebiotics are supplements of specific gut microbiota species.
Fun fact, poop is generally more than 50% excess/dead gut microbiota cells. And there is some fibre and waste in there, but mostly its your little gut flora and fauna.