Table 1. Rat density and rat's virus carriage rate in cities of Hebei province from 1999 to 2011.

	Rat density		Rat's virus carriage rate
Year	Residential area (%)	Field area (%)	Residential area (%)	Field area (%)
1999	6.02	9.08	3.69	5.43
2000	5.4	8.53	3.14	8.79
2001	4.86	10.08	2.49	6.12
2002	3.99	7.71	1.92	5.2
2003	3.37	2.23	3.89	4.27
2004	3.38	1.89	3.39	3.06
2005	1.48	0.99	4.62	0.97
2006	2.08	1.19	5.24	1.18
2007	2.18	1.36	2.82	1.44
2008	2.62	1.24	3.63	1
2009	2.02	0.52	3.02	0.89
2010	2.08	1.27	1.96	0.84
2011	2.01	0.7	7.46	0