But he also reveals why rational people might have become caught up in it. The flowers had unique color patterns much in demand for their beauty, but each new variety had to be propagated from a single bulb which could only produce two bulbs in the next year, four after that, and so on, When the available quantity was small, naturally the underlying value of a single bulb increased. The more abundant varieties sold cheaply by the pound. To complicate matters, the gorgeous markings on the most striking bulbs were actually the result of a virus. That made them sickly and difficult to propagate. This biologically-determined rarity added to their value and kept price higher than would normally be expected. Until 1634 or so, tulip prices behaved normally, with rare, slowly propagating varieties more expensive than the plentiful varieties.