Widal test is used for the diagnosis of

Typhoid fever is caused by the bacterium $Salmonella\ typhi$, which enters the small intestine through contaminated food and water.
Its symptoms are sustained high fever (39–40 °C), weakness, stomach pain, constipation, headache and loss of appetite.
Typhoid fever is confirmed by the Widal test, which detects antibodies against the pathogen in the patient's serum.
Malaria is diagnosed from blood smears, and pneumonia and tuberculosis by other tests (e.g., X-ray, sputum test, Mantoux test).
Hence the Widal test is used for the diagnosis of typhoid.