The correct answer is option 2: bacteria. Typhoid fever is caused by the bacterium Salmonella typhi. It is an infectious disease that is primarily transmitted through contaminated food and water. The bacteria enter the body through the mouth and intestines, and then invade the bloodstream. Symptoms of typhoid fever include high fever, headache, stomach pain, constipation or diarrhea, and a characteristic rash. If left untreated, it can lead to serious complications and even death. It is important to note that typhoid fever is not caused by a virus, fungus, or allergy.