The disease was first described in 1900 by investigator Rettger, who isolated the organism from infected chicks and named it Bacillus pullorum . It is caused by Salmonella enterica serovar Gallinarum biovar Pullorum, a gram-negative, non-motile bacterium. While it has been largely eradicated from commercial poultry in many developed countries through rigorous testing and culling programs, it remains a significant threat in backyard flocks and some developing regions. Transmission: The Cycle of Infection
Infected "carrier" hens harbor the bacteria in their ovaries. The bacteria pass into the egg during formation, meaning chicks are born already infected.
Within a brooder or hatchery, the disease spreads rapidly through contact with infected droppings, contaminated feed, water, or even through the air in forced-draft incubators. Clinical Signs and Diagnosis Bacillary White Diarrhea - ScienceDirect