Yeast: A group of single-celled fungi that reproduce by budding.
Most yeast are harmless (some are used in baking and brewing). Yeast is commonly present on normal human skin and in areas of moisture, such as the mouth and vagina, usually without causing any problems.
However, yeast can produce disease in people. For example, the yeast Candida (once called Monilia) causes thrush (oral infection) and diaper rash in infants, fingernail infections, vaginal area infections in women after puberty, and a host of problems in patients with immune deficiency.
Yersinia pestis: The bacteria that causes the bubonic plague which in the year 541 (as the Black Death) and later in the Middle Ages decimated Europe. Yersinia pestis mainly infects rats and other rodents which are the prime reservoir for the bacteria. Fleas are the prime vectors carrying the bacteria from one species to another. They bite rodents infected with Y. pestis, then they bite people and so transmit the disease to them.
Transmission of the plague to people can also occur from eating infected animals such as squirrels. Once someone has the plague, they can transmit it to another person via aerosol droplets.