The Widal test is a blood test that is used to detect antibodies to the Salmonella typhi and Salmonella paratyphi A bacteria, which cause typhoid fever and paratyphoid fever, respectively. The Widal test measures the levels of antibodies to the O and H antigens of the bacteria. The O antigen is a protein found on the outer surface of the bacteria, while the H antigen is a protein found on the flagella of the bacteria. A positive Widal test result means that the person has been exposed to the bacteria, but it does not necessarily mean that they have an active infection. A positive test result should be followed up with a more specific test, such as a blood culture, to confirm the diagnosis of typhoid fever or paratyphoid fever. The Widal test is a simple and painless blood test that can be performed in a doctor's office or a laboratory. The results of the test are usually available within a few days.