Pasteurella pestis, also known as Yersin's bacillus, is a rod-shaped gramnegative bacterium which causes the plague. Its reproduction is slow in ordinary cultures and needs high humidity rates and oxigen to multiply, in 25-30º C temperatures. The Pasteurella pestis secrets endotoxins which attack either the lymphatic system or the lungs. It is transmitted to man through an insect vector: the rat's flea. It was discovered by Alexandre Yersin in 1894.