West Nile virus arrives in Massachusetts (2000-2001)
West Nile virus, first found in New York in 1999, reached Massachusetts mosquitoes in 2000. The state confirmed its first human infections in 2001, marking the start of an annual mosquito-season threat.
Health officials established permanent mosquito surveillance and spraying programs.
More than 275 Massachusetts residents have been infected since 2001, with deaths occurring most years.
This is the baseline for today's story — this year's cases are part of a 25-year pattern, not a new disease.
