Federal health officials warned Thursday that the United States could be on the verge of a major outbreak of measles.

The official tally of measles cases between Jan. 1 and April 25 was 64, the highest in recent years, officials from the Centers for Disease Control and Prevention (CDC) said.

That count doesn't include Washington state, where eight cases were reported this week. Those cases stemmed from an international church conference in suburban Seattle in March, according to the state health department.

Most of the cases have been traced to outbreaks overseas and are mainly in children who were not vaccinated for religious or other reasons or were too young, according to the CDC. Since measles vaccinations began in the early 1960s, cases have dramatically declined in the U.S.

While the numbers seem small, two developments could set the stage for a major resurgence in this country: an increase in the numbers of people choosing not to get vaccinated and outbreaks of the disease in Israel and Europe, CDC officials said.

"I think the principal difference this year is the extent of outbreaks in Europe," said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases.

"The second factor is ... we are in a very different era right now where many doctors, nurses and parents are not familiar with measles, where people can seek medical care for something and transmission can occur because we don't take all the precautions that we would have."

The 64 cases this year were in nine states. There were no deaths, but 14 people were hospitalized, CDC spokesman Curtis Allen said.

U.S. residents accounted for 59 of the 64 cases.

Officials think all but 10 cases were either acquired overseas or linked to a chain of transmission that originated in another country.

Measles is caused by a virus that normally grows in cells that line the lungs and the back of the throat. It spreads through contact with a sneezing, coughing, infected person.

Symptoms include rash, high fever, cough, runny nose and red, watery eyes. But about 1 in 5 measles patients experiences illness that can include diarrhea, ear infections, pneumonia, encephalitis, and even seizures and death.

Thirteen of the U.S. cases were children younger than 1; children usually don't get their first measles shot until they're at least 1 because their immune systems are considered too immature to produce the needed response.

Such children can easily pick up infections from those around them, said Dr. William Schaffner, chairman of Vanderbilt University's department of preventive medicine.

This year's measles cases are the most since 2001, when 116 cases were reported, according to CDC records.

The worst year for measles was 1958, according to modern public-health records. More than 763,000 cases were reported that year, including 552 deaths. Outbreaks in the early 1990s led to a revision of vaccination guidelines to include children younger than school age.

The measles vaccine is 99 percent effective.