The Widal test is commonly used to diagnose enteric fever, caused by *Salmonella typhi*. It detects specific agglutinating antibodies (O and H antibodies) against the bacterial antigens in the patient's serum. A significant rise in antibody titers indicates recent or ongoing infection. This test is particularly useful in endemic areas and aids in supporting clinical diagnosis when blood cultures are not readily available.

Reference: Harrison's Principles of Internal Medicine, Kasper et al., 20th Edition.