Mid-West Nile Outbreak: Day 6 part 2

Ok, it seems there have been a few more fatalities associated with West Nile.  ABC News just reported that there have been West Nile related deaths both in Dallas Texas, and Fresno California.  It seems the victims were both elderly.  It really begs the question:  Why has this strain become so virulent, so quickly?  This wasn’t even news a week ago and now several confirmed cases all across the country?