Illustration showing the injection mechanism of the Yersinia pestis toxin into macrophage immune cells in three steps. Step 1 shows a part of the injection system (red) and the Yersinia pestis toxins (green) within the bacterium. Step 2 shows the perforation of the macrophage cell membrane prior to the injection. Step 3 shows the transfer of the Yersinia pestis toxins into the target cell. Yersinia pestis bacteria causes the plague, which is responsible for the deadliest pandemic in recorded history.