Tiny and very dangerous, the pill shapes among the yellow-green goo are Yersinia pestis, the bacteria that cause the bubonic plague. They are shown here, thanks to an electron microscope, inside the guts of a flea. Although the disease is now rare among humans and can be treated with antibiotics, the plague killed over 25 million people during the Middle Ages. Fleas spread the plague bacteria to humans by biting. (Image via NIAID) Read More