The mumps epidemic in Iowa continued to widen this week and reached 605 cases by Thursday, public health officials said.

Although the highest concentration of cases remained in eastern Iowa, the virus that causes mumps has infected at least one person in half of the state's 99 counties.

"I certainly would consider this a serious threat," said Dr. Patricia Quinlisk, the state's epidemiologist. "We're doing everything we can to try to address it, get information out, do what we can to try to get it under control."

The state has received about 50 reports of new infections a day, a level which has remained consistent for at least a week. As of Monday, Nebraska has 43 reported cases; Kansas, 33; Illinois, four; Missouri, four; Wisconsin, four; and Minnesota, one.

Iowa Department of Public Health Director Mary Mincer Hansen said questions about how the epidemic started and why it has hit Iowa so hard have not yet been answered.

"I would guess that somehow we were unfortunate enough to be in the place where the mumps virus was introduced in such a way that it could cause an outbreak," she said. "After the outbreak has gone away, we'll look back at all the data we're collecting and try to piece together what could have been some of the causes."

Hansen said the mumps outbreak has been a good test of the state's public health systems established after the Sept. 11, 2001, terrorist attacks. Those systems include a health alert network, a bioemergency plan and education of employees at local and state agencies on how to respond to outbreaks.

"We do feel we could have a worse problem if we didn't have the resources already available here in Iowa," Hansen said.

She acknowledged that the systems have not succeeded in stopping the spread of the highly contagious mumps virus, and said the state's response has included seeking further advice from the U.S. Centers for Disease Control and Prevention. CDC employees were in Iowa Thursday helping review data and plan prevention and control strategies.

"We always are looking for things we could do differently, which is why we invited the CDC to come here and talk with us," Hansen said.

Quinlisk said a quarantine of infected people was ruled out.

"We have discussed that with the experts and everybody agrees that particular strategy would just not work in this disease," she said.

Instead, the state is using an isolation strategy, which includes encouraging people with symptoms to stay home.

Mumps is a virus-caused illness spread by coughing and sneezing. The most common symptoms are fever, headache and swollen salivary glands under the jaw. But it can lead to more severe problems, such as hearing loss, meningitis and fertility-diminishing swollen testicles.

No deaths have been reported from the current epidemic.

A large portion of those with mumps are college-age and few are school-age or preschool children. Health officials said that's likely because of the very high vaccination rates among children.

Even with the widespread vaccination, the mumps vaccine is 95 percent effective, meaning just five in 100 people vaccinated will not develop antibodies to the virus and can contract the disease.

About a quarter of the Iowans who have suspected cases got the vaccine, health officials have said.