(CNN) -- Sixty-six people have died from West Nile virus infections this year, and the number of human cases has grown to 1,590, the U.S. Centers for Disease Control and Prevention said Wednesday.
That's the highest case count through the last week of August since the virus was first detected in the United States in 1999.
Nearly half of all the infections have occurred in Texas, where officials said later Wednesday that 894 cases have been reported along with 34 deaths.
"Those numbers are going to go up," said Dr. David Lakey, commissioner for the Texas Department of State Health Services.
Lakey said it looks like 2012 will be the worst year so far when it comes to West Nile virus cases. In 2003, Texas reported 40 deaths because of the virus, and health officials believe they will surpass that number this year.
All lower 48 states are now reporting West Nile activity, and 43 states have reported at least one person infected with the virus.
More than 70% of all West Nile virus cases in the United States are found in six states: Texas, South Dakota, Mississippi, Oklahoma, Louisiana and Michigan, according to the CDC.
While the CDC said these are the highest number of cases reported by the last week in August since the disease was first recorded in 1999, it's not the highest number the country has seen. In 2003, the United States had 9,862 reported cases of West Nile virus, and in 2002 there were more than 4,100 cases and 284 fatalities.
In more than half the current nationwide cases, West Nile has led to neuroinvasive disease -- serious illnesses like meningitis, encephalitis or virus-associated paralysis. About 10% of these cases can be fatal, according to the CDC, and a high proportion of those who survive can be left with longstanding neurological problems.
Older people are more vulnerable to severe illness from West Nile. So far, the median age among those with neuroinvasive disease this year is 58. In Texas, nearly two-thirds of all cases are in people older than 50.
Health officials do not yet know why there are more cases this year than in recent years.
Based on previous experiences with floods and hurricanes, health officials do not believe Hurricane Isaac will have a major impact on this year's outbreak.
Mosquitoes that spread the virus breed in small nutrient-rich pools of water, such as the water found in old tires, so a large rainfall event or flooding usually washes out those small pools and eliminates breeding sites, said Dr. Lyle Petersen, who heads the CDC's Division of Vector-Borne Infections.
But after the water recedes, there's always the possibility that new small pools of water will form.
Petersen urged the public to take proper precautions to prevent getting sick regardless of whether they live in a state with high or low case counts.
To reduce your risk of exposure to mosquitoes and to prevent their breeding sites:
-- Drain all standing water from flower pots, old tires, clogged rain gutters, etc., where mosquitoes breed.
-- Use an insect repellant that contains DEET, picaridin or oil of lemon eucalyptus.
-- Also wear long sleeves and long pants to protect your skin when you're outside.
-- Stay indoors at dusk and dawn because that's when mosquitoes are most active.