Plague is an infectious disease caused by the Yersinia pestis microorganism, 
which is transmitted to the human host from a natural reservoir (different 
rodent species) by a flea bite. Plague is still encountered in humans in the 
areas of its enzootic prevalence in local rodent populations. Infection by flea 
bite results in a bubonic or septicemic plague, possibly complicated by 
secondary pneumonia. The person with pneumonic symptoms may be a source of a 
droplet-borne inhalatory infection for other people who consequently develop 
primary pneumonic plague. Despite a clinical form, plague is a severe infection 
characterized by a short incubation period, rapid onset and quick progress with 
mortality exceeding 50% if not treated properly. The pneumonic plague is 
associated with a particularly rapid progress and the mortality rate of almost 
100% if not treated properly. As Yersinia pestis can be easily obtained and 
cultured and is highly pathogenic for humans, it poses a serious threat of being 
used for bioterrorism purposes. Artificially created aerosol containing plague 
bacilli can cause numerous and almost simultaneous cases of primary pulmonic 
plague in an exposed population. Persons exposed would most likely develop 
severe pneumonia with rapidly progressing respiratory and circulatory failure. 
The use of the Yersinia pestis strains resistant to antibiotics typically 
applied cannot be excluded.