Table 1. Period of isolation and source of Escherichia coli isolates tested, United States, 1950–2002.

Period	Source, no. isolates
	Human	Cattle	Chickens	Pigs
1950–1959	180	0	0	0
1960–1969	112	17	0	15
1970–1979	292	32	11	25
1980–1989	211	86	65	96
1990–1999	182	108	44	81
2000–2002	6	81	18	68