Tzaraas (discussed in Vayikra 13-14) is a human skin disease that renders its victim impure and subjects him to many laws. In a different form, tzaraas can also affect clothing and houses. It is generally understood to be punishment for certain sins, such as talebearing.
As far as I'm aware, tzaraas doesn't occur nowadays. But during the era that it did happen, how common was it? Was it a once-in-a-century type thing? Or was it more like if you had a nasty neighbor, you could be pretty sure he'd soon get tzaraas? Were some of the three types more common than others?