The causative agent of typhoid fever is Salmonella. Salmonella typhi is the specific bacterium responsible for causing typhoid fever, which is a bacterial infection that is spread through contaminated food or water. The bacteria can survive in water or soil for several weeks and can be transmitted from person to person through contact with feces or urine of an infected individual. Symptoms of typhoid fever include fever, headache, stomach pain, and diarrhea. It is important to seek medical treatment if you suspect you have typhoid fever as it can lead to serious complications if left untreated.