I've had the exact opposite experience in my rescue. Cultures and/or bloodwork confirm RI in many snakes that the owners never even suspected had a problems, they just knew the snake was acting listless.
But not all infections are bacterial, and so they don't all respond to antibiotics. Aspergillosis, a fungal infection, is a leading cause of RI in snakes kept on organic substrates such as mulches, bark, peat moss, or soil. Nearly 1/3 of the snakes I've treated for RI had aspergillosis.
The Zombie Mama is here!