As a result, tulips rapidly became a coveted luxury item, and a profusion of varieties followed. They were classified in groups: the single-hued tulips of red, yellow, or white were known as "Couleren"; the multicolored "Rosen" (white streaks on a red or pink background); "Violetten" (white streaks on a purple or lilac background); and the rarest of all, the "Bizarden (Bizarres)", (yellow or white streaks on a red, brown or purple background). The multicolor effects of intricate lines and flame-like streaks on the petals were vivid and spectacular, making the bulbs that produced these even more exotic-looking plants highly sought-after. It is now known that this effect is due to the bulbs being infected with a type of tulip-specific mosaic virus, known as the "tulip breaking virus", so called because it "breaks" the one petal color into two or more.