Plague

Blackened skin. Swollen lymph nodes. Rats and other pests. Fleas. Ew.
You know what I'm talking about. The Plague.
It's caused more human deaths than any other infectious disease, save for malaria and tuberculosis.[1]

It's caused by the bacteria Yersinia pestis, a gram-negative, rod-shaped bacterium.
It's primary host is the rat, and the intermediate hosts between rats and humans are fleas. (Guess that gives me another reason to lay down rat/mouse traps.)
Since it primarily infects rats, but sometimes infects humans as well, it is a zoonotic disease. (disease which primarily infects animals, although it can also cause disease in humans)
Infected rats generally die soon after infection, but the survivors develop a chronic condition in which the animal can keep transferring the disease to other organisms.[1]

Alright, my little pet hamster doesn't carry fleas,
but that isn't the case for every pet rodent..

Yersinia pestis can cause multiple forms of plague:
- Sylvatic plague
- Bubonic plague
- Septicemic plague
- Pneumonic plague



Forms of plague
Sylvatic plague
Plague in wild animals (especially wild rodents)[1]
Sylva or Silva translates to "wood" or "forest" in Latin.

Bubonic plague
Once the bacteria enters the human body and travels to the lymph nodes, it causes the swelling of the lymph nodes. Swollen lymph nodes are then called 'buboes', hence the name 'bubonic plague'.
Yersinia pestis is remarkably hard to destroy because they have several antigens which inhibit phagocytosis. The bacteria can then travel to other lymph nodes and eventually enter the bloodstream. If left untreated, bubonic plague could advance to septicemic stage.[1]

Septicemic plague
As some of you might or might not know, 'Sepsis' is a condition in which inflammation occurs all over the body as a response to certain diseases or injuries. 'Septicemia' is when a bacteria in the blood causes sepsis.
Once Yersinia pestis enters the bloodstream, it rapidly spreads throughout the body. It does not have to cause the formation of buboes if it directly enters the bloodstream.
It causes hemorrhages on the skin, making it look black. This is why it's called 'Black Death'.
If left untreated, death usually occurs within 3-5 days.
Patients usually die even before a diagnosis can be made.[1]

Pneumonic plague
Occurs if Yersinia pestis is inhaled or if it reaches the lungs via the blood or lymph. It doesn't cause any symptoms for the first 2-3 days, until the patients start producing large amounts of bloody sputum. Because it's easily spread by coughing, infected individuals should be immediately quarantined.
It's very hard to treat, and even if treatment is started in time, 9 out of 10 patients die.[1]


Use in biological warfare
Bioterrorism agent category: A, highest category possible.
It's remarkably easy to use and deliver, and it's a good killer. What more could you want?[1]

Treatment
Antibiotics: Streptomycin, gentamicin, doxycycline, ciprofloxacin, chloraphenicol[1]

Cases/year
Worldwide: approximately 2000[1]

Vaccine 
Vaccine available: Yes (killed/inactive vaccine, cell extract vaccine)[1]


[1] Madigan, M., Martinko, J., Stahl, D., Clark, D., Brock Biology of Microorganisms, 2011, Pearson