Many arboviral diseases are characterized by long periods of invisibility, when little or no evidence of their existence can be detected. At erratic intervals, sometimes separated by several decades, there is a sudden recrudescence, often developing into an explosive epidemic. Retrospective studies of such epidemics frequently suggest, by association, weather-related factors which could have been responsible for triggering this recrudescence, and these associations offer fertile ground for speculative explanations. Recrudescence is by no means unique to arboviral infections. In the aftermath of widespread transmission of all viral diseases, the immune system acts within populations as a common defense factor, the herd immunity, which restrains the recurrence of high rates of transmission until a sufficiently large number of nonimmune individuals are recruited, either by natural increase or by immigration from other populations. Recurrent epidemics of measles and influenza viruses are classic examples of this mechanism.