Plague infected oriental rat flea (Xenopsylla cheopis) engorged with blood 12 days after feeding on a mouse inoculated with Yersinia pestis, the bacterial cause of the plague. This flea may also feed on humans, spreading the disease. In humans infection is rapid, causing swollen lymph nodes and leading to septicaemia, which is always fatal, and pulmonary infection, which can be fatal within a day. Prompt treatment with antibiotics can effect a cure.