Nearly 3,000 severe illnesses from West Nile and more than 260 deaths occurred each year in 2002 and 2003, according to CDC's top West Nile expert, Dr. Lyle Petersen.
About half of the cases of 2012 are serious illnesses, and the CDC considers those the best indicator of West Nile activity because many mild cases do not get reported and their symptoms may not even be recognized.
Health officials think that West Nile activity will peak in mid-to-late August, but likely will continue through October. Because symptoms can take two weeks to appear, reporting cases lags behind when people became infected.
All states except Alaska and Hawaii have found West Nile virus in people, birds or mosquitoes this year. Texas has been the hardest hit, accounting for half of the cases reported to the CDC so far. The disease first appeared in the United States in 1999.
Typical symptoms are fever, headache and body aches, and most people get better on their own in a few days. Less than 1 percent develops neurological symptoms such as stiff necks and even coma and paralysis.
The Associated Press contributed to this report.