Plague is an ancient scourge of mankind. In ancient days, plague is believed to have killed a quarter of the mankind. So it was known as ‘black death’. In fatal cases, it also caused extensive cutaneous hemorrhage and gangrene.
Plague bacillus is a short, plump, ovoid bacillus, called Yersinia pestis. It is an aerobic bacterium. It grows well at 270C. The bacteria can resist cold and moist environment, but easily destroyed by heat at 550C, sunlight, drying and chemical disinfectants. It can survive for several months and multiply in the soil of rodent burrows. The bacteria possess about 20 antigens. Many of them are responsible for the virulence of the bacteria. They include;
1. A heat labile protein envelope antigen
2. Two antigens designated V and W which are produced together and inhibit phagocytosis.
3. Virulent strains produce a bacteriocin, coagulase and fibrinolysin.
4. Virulence also appears to be associated with an unidentified surface component.
5. Virulence has also been associated with the ability for purine synthesis.
6. YOPS; Yersinia plasmid encoded outer membrane protein which help the bacteria to enter inside the phagocytic cells and to multiply inside the cells.
    
    In humans, plague occurs mainly in three forms;
1. Bubonic; the lymph nodes at the site of entry of the bacillus become infected. This occurs after 2-5 days of incubation. The inguinal nodes are involved, as the bacilli enter through flea bites on the legs. Hence the name bubonic (‘bubon’ means groin).  The glands enlarge and the bacilli enter the bloodstream causing septicemia. Gangrene of the skin, fingers and penis occurs. Hemorrhage of the skin and mucosa also seen.
2. Pneumonic; is spread through infected droplets. The bacteria enter through the lungs and cause hemorrhagic pneumonia. Certain patient’s seem to develop blueness of skin due to insufficient oxygen. Bloody mucoid sputum is coughed out carrying large number of bacilli in them and acts as a source for spreading the infection. It is fatal in untreated cases.
3. Septicemic; rarely results in meningitic involvement. Asymptomatic oropharyngeal infection is observed in some patients.

Plague is termed as ‘zoonotic’, as it involves an animal. Plague bacillus is a natural parasite in rodents and transmitted among rats by rat fleas known as Xenopsyla cheopis feeding on infected rats. The bacteria grow inside the fleas to such an extent that it blocks the passage to the intestine mechanically. When such a ‘blocked flea’ bites another rodent, instead of sucking in blood, the bacteria are injected back into the rodent. Infection may also be transmitted by contamination of wounds with the feces of infected fleas. When such an infected rat bites a human, bubonic plague is contracted.

Two natural cycles of plague subsist; domestic and wild. Domestic plague is one that is obtained from the rodents that are very closely associated with the humans. Wild plague occurs in wild rodents independent of human beings. It may be transmitted to humans while handling the dead body of the infected rats or during skinning. Clinical plague may be seen in cats also and can spread to human through droplets.

Control of rodents and fleas are the most important way of prevention. Vaccination gives short term protection. Chemoprophylaxis with cotrimoxazole or tetracycline is also recommended. Antibiotics like streptomycin, chloramphenicol and gentamycin are found to provide effective treatment.