ATLANTA -- The number of West Nile virus cases has tripledin just one week and a U.S. health official warned Thursday thatthis year's outbreak may top last year's record.

WEST NILE VIRUS Interactive: All About West Nile Overview: Learn About Disease Protection: Insecticide Tips More: Virus In Horses RESOURCES Encyclopedia: West Nile Defined Facts: CDC | Q&A Maps: Cases By State | Cases By County Phone CDC Hot Line: (888) 246-2675

"The numbers are starting to change very, very quickly," saidDr. Julie Gerberding, head of the Centers for Disease Control andPrevention. "That is very concerning."

So far, there are more than 153 cases in 16 states, an explosionof the mosquito-borne virus in just a week, she said. That doesn'tinclude 39 new cases Colorado officials reported Thursday.

Colorado's total now stands at 111, with four deaths -- the most of any state in the United States.

A week ago, there were 59 cases nationwide. Health officials hadexpected the disease to spread this year, invading Western statespreviously unscathed. But they appeared somewhat surprised at itsspeed.

The new numbers compare with 112 cases in four states for thisdate last year.

"It indicates we are starting the epidemic with more cases thanlast year," the CDC director said. She warned of "a great numberof infected people."

Last year, 4,156 people caught the virus, and 284 died. TheUnited States also suffered the biggest reported outbreak of WestNile encephalitis in the world in 2002.

West Nile virus rarely kills, but about one in 150 people who getit will develop its potentially deadly encephalitis or meningitis.Most often, it affects the elderly. Of the seven people who havedied this year, the youngest was 68.

Four of the deaths have been in Colorado, the hardest hit statewith at least 111 cases. Officials blame the the outbreak ona wet June and extremely hot July, which they say provided theperfect summer for mosquitoes.

"I can't predict what will happen in Colorado, nor can Icompletely explain why it is happening," Gerberding said.

Last year, that state had about a dozen cases. Four states _Arizona, Utah, Nevada and Oregon _ had no signs of the disease inman or animal last year.

"If it can increase that dramatically in Colorado, it has thepotential to do so in Arizona," said Craig Levy of the ArizonaDepartment of Health Services.

Until Colorado's first death a week ago, the virus had neverkilled anyone west of Texas, Oklahoma, Nebraska and North Dakota.

Since West Nile virus first entered this country through New York in1999, health officials have tried everything _ mosquito sprayingand other control efforts, prevention messages and diseasedetection systems.

But there's no way to prevent the virus from spreading andthere's no way to predict which areas it will strike hardest, saidDr. Sue Montgomery of the CDC.

Last year, Louisiana had more than 300 cases and 25 deaths fromthe virus. They did "everything ... according to the book and wehad a large epidemic," recalls Dr. Raoult Ratard, stateepidemiologist for the Louisiana Department of Health andHospitals.

It's like a viral hurricane," Ratard said.

Most people who are infected with the virus won't get sick. TheCDC says about a fifth of those will develop a fever, headache,body aches and sometimes a rash and swollen lymph glands.

Symptoms for West Nile encephalitis or meningitis includeheadache, high fever, neck stiffness, disorientation and sometimesparalysis.