Fowl typhoid and pullorum disease are among the most important diseases of poultry. These conditions are caused by two very closely related organisms, which were once thought to be different species but have recently been classified as biovars of Salmonella enterica subsp. enterica. Pullorum disease is usually symptomatic only in young birds. The mortality rate varies, but it can be as high as 100%. Fowl typhoid resembles pullorum disease in young birds, but it is also a serious concern in growing and adult poultry. The control of these diseases is complicated by vertical transmission: hens can become subclinically infected carriers, and pass the infections to their embryos in the egg. Fowl typhoid and pullorum disease have been eradicated from commercial poultry in many developed countries including the United States and Canada, but they may persist in backyard poultry flocks and game birds. Pullorum disease is an increasing concern in pheasant chicks. On rare occasions, these diseases have been reintroduced to commercial chicken or turkey farms.