The 20th century emergence in Massachusetts of zoonotic eastern encephalitis was interpreted in terms of recorded environmental change. The main mosquito vector of the infection, Cs. melanura, appears to have been scarce in eastern North America before the 1930s. Its relative scarcity resulted from destruction of the swamps that had been lumbered or drained for farming in the 18th and 19th centuries. When swamps matured once again early in the 1900s, the formation of subsurface pools of water beneath mature trees would have increased the availability of breeding sites for this mosquito. Transmission would have further been enhanced by the simultaneous proliferation of wetland-roosting robins and the extinction of such vagile birds as the passenger pigeon. Although numerous horses were maintained in Massachusetts at the time, no outbreaks of "equine sleeping sickness" came to public notice between the 1830s and the 1930s, when mature trees were scarce and the fauna was most disturbed. The severity of the first major outbreak in 1938 may have been potentiated by the absence of herd-immunity in a rapidly proliferating population of reservoir birds. These considerations suggest that recent landscape and faunal changes caused zoonotic EE to emerge in Massachusetts after waning for a century.