West Nile virus in humans, 2,002. From the USGS website.
They have other maps showing bird, mosquito and veterinary spread of WNV but this one looks like outbreaks are centered around large urban populations for the most part. Maybe that's just 'where the humans are' but LA for one seems an odd place.
That map is interactive on the website. State maps can be had by clicking on it. Also, the amount of red on the map doesn't mean a lot. One positive case in a county would result in the whole county being shown in red whereas a relatively small county like Cook co., IL which has had 309 cases makes no bigger impression on this map than if it had one.