The Plague of Justinian I began in Ancient Egypt, and spread to the Byzantine Empire, where it got its name from the emperor at the time, Justinian I. The cause of the plague is the black rat, which happened to travel on the grain ships sent to Constantinople as a tribute offering. These rats carried fleas, which were infected with Yersinia pestis, otherwise known as the Black Death. Despite the name being singular, there are 3 types of the Black Death. They are: Bubonic, Septicemic, and Pneumonic. Each of the three types have different symptoms and infection sites. This plague spread so fervently because the Byzantine Empire were trying to expand their reach, meaning war-torn villages and towns. The weather didn't help either, with a 20+ year long cold-snap just beginning in Italy. With these factors combined, it is easy to see how a pandemic could have sprouted from these conditions.
Here's a list of symptoms and infection types/styles. The Bubonic plague causes painful bumps, swelling, fever, and vomiting, and is an infection of the lymph nodes. Septicemic plague is an infection of the blood that causes gangrene, sepsis, and disseminated intravascular coagulation, which is the clotting of blood in small blood vessels throughout the body. It is the rarest form of the Black Death, but is almost always fatal if contracted. Lastly, Pneumonic plague is an infection of the lungs, which causes fever, shortness of breath, chest pain, and a cough. Pneumonic plague can occur after an initial bubonic or septicemic plague infection, making it more dangerous than the other two. Plus, it's airborne, meaning you can breathe in microdroplets of plague without realizing it.