When a person becomes infected with the plague bacteria, they begin to multiply within the body. For bubonic plague, this occurs in the lymph system; for pneumonic plague, this occurs within the lungs. The lymph, or lymphatic system, is a major component of your body's immune system. The organs within it are the tonsils, adenoids, spleen, and thymus.
Between one and six days later, symptoms of plague usually begin. The period between becoming infected and the start of symptoms is called the incubation period.
Symptoms are different for each type of plague. It is possible for a person to have symptoms of only one type; it is also possible for a person to experience symptoms from each of the types. For example, a person may first develop signs of bubonic plague, followed several days later by septicemic plague symptoms, and then signs of pneumonic plague.
In order to diagnosis someone as being infected with plague bacteria, the doctor will ask a number of questions about a person's medical history and perform a physical exam. During the exam, the doctor will listen to the lungs and look at the skin to check for signs of infection. If the doctor has a high suspicion that a person is infected, he or she will recommend certain tests.
Because several conditions share some similar symptoms of plague, the doctor will check for these and rule them out before making a firm diagnosis. These conditions can include:
- Lymphogranuloma vernereum (a sexually transmitted disease affecting the lymph system)
- Syphilis (a sexually transmitted disease)
- Tularemia (a serious illness usually caused by animals)
- Typhoid fever (a life-threatening illness caused by Salmonella)
- Shigellosis (an infectious disease typically caused by unsanitary conditions)
- Cat scratch fever (a disease associated with being scratched by a cat).