It must be your regional perception. I had not heard of West Nile Virus until we moved to Colorado. Our second summer here was terrible, many people infected and quite a few died.
2003.
After that, targeted mosquito spraying was implemented, over the cries of the nutsos, and our infections plummeted.
I researched a few CDC charts, and you can see the same thing happen in other states. 2004 - CA had massive infections, then they dropped off.
They dropped off because a fair amount of the vulnerable population was affected the first time through. Then the incidence dropped to a low level background rate. The bow wave of an infection through a naive population is always scary. Thats how ebola will be if it gets out widely.