Accumulated evidence about Kyasanur Forest disease (KFD) is reviewed in an effort to explain why outbreaks have been confined to the Sagar-Sorab area of Mysore State. India. Immunity surveys carried out as early as 1952 and on later occasions indicate that KFD virus (or some closely related agent) has been active inconspicuously in various other regions of India, of contrasting ecology and widely separated from each other. The probabilities that the virus may have been imported into the Sagar area are considered and evaluated as low-grade. Several species of small mammal in the Sagar forest have been found capable of multiplying KFD virus in their systems, and the virus has been detected in tick parasites of the genera Haemaphysalis and Ixodes. Adult ticks have been found attached to sick and dead monkeys. Haemaphysalis have been observed in nature to carry the virus in nymphal and adult instars for up to 14 months, spanning three seasons. Adults have been shown experimentally to be efficient transmitters of the virus. Experimental exposure of monkeys to the bite of tick larvae in the forest suggests that monkeys well may serve as efficient amplifiers in the KFD-virus cycle. Man's low rate of attack by H. spinigera is offset and the probability of his infection increased by increased numbers of infected vectors. Since 1951 the human population of the Sagar-Sorab area has more than doubled, and the consequent alterations of the ecosystem have produced a set of conditions favoring overt expression of a hidden enzootic process.