Time-space clusters of West Nile virus (WNV) plus dead birds (dots) and WNV+ mosquito pools (triangles) in Los Angeles County. The first evidence in the Los Angeles Basin in 2003 (A) with mosquito pool cluster a (10/9/2003–10/29/2003), and dead bird clusters 1 (9/13/2003–10/15/2003) and 2 (10/30/2003–12/3/2003). Amplification in the Los Angeles Basin in early 2004 (B) depicted by dead bird clusters 1 (5/8/2004–6/4/2004) and 2 (5/29/2004–7/2/2004) overlapping with mosquito pool clusters a–c (6/26/2004–7/30/2004, 7/3/2004–8/6/2004, and 7/10/2004–8/13/2004), then progressing into the San Fernando Valley in late 2004 (C) dead bird cluster 3 (7/10/2004–8/13/2004), and mosquito pool clusters d (7/24/2004–8/20/2004) and e (7/24/2004-8/20/2004). Amplification in Santa Clarita in 2005 (D) with dead bird clusters a (7/24/2005–6/27/2005) and b (9/11/2005–10/8/2005).