By Debra Goldschmidt, CNN

(CNN) -- The number of cases of measles in the United States this year has surpassed 1,000, the US Department of Health and Human Services said Wednesday. The agency said there have been 1,001 cases so far this year.

That's 20 more cases than the US Centers for Diseases Control and Prevention reported on Monday in its weekly national update. This year, which is barely half over, has the greatest number of cases in a single year in nearly three decades.

"The Department of Health and Human Services has been deeply engaged in promoting the safety and effectiveness of vaccines, amid concerning signs that there are pockets of undervaccination around the country," said US Health and Human Services Secretary Alex Azar said in a statement.

"The 1,000th case of a preventable disease like measles is a troubling reminder of how important that work is to the public health of the nation," Azar continued.

The number of cases this year is the highest since 1992 when there were 2,237 cases of the highly contagious illness reported in the United States. It's the highest number of cases in a single year since it was eliminated nationwide in 2000, meaning it was no longer continuously transmitted in the country.

Cases have been reported in more than half of US states. New York has been the largest contributor to this year's unfortunate milestone with nearly 700 cases of measles reported this year in the state.

Most of those cases have been in Orthodox Jewish communities In Brooklyn and Queens with low vaccination rates. The New York City Health Department reported that as of June 3 there had been 566 confirmed cases in those areas since September.

Clark County in Washington state had the second-largest outbreak in the US this year with more than 70 cases reported.

Highly contagious

Measles is one of the most highly contagious diseases in existence, spreading through coughing and sneezing, and can linger in the air for up to two hours. If someone who is not immune to the virus breathes the air or touches an infected surface, they can become infected, according to the CDC.

Once a person has measles, about 90% of close contacts who are susceptible to it will develop the disease. Early on, measles can be confused for other viral illnesses such as the flu. But the red blotchy rash that comes with it may help set it apart.

The virus often manifests as a combination of high fever -- as high as 105 degrees Fahrenheit -- along with cough, runny nose and pink eye, according to Dr. Julia S. Sammons, a pediatric infectious disease specialist and medical director of the Department of Infection Prevention and Control at the Children's Hospital of Philadelphia.

To protect yourself, doctors recommend immediate vaccination. Other steps: wash hands often or using sanitizer, avoid touching your eyes and mouth, disinfect surfaces and toys with standard household products, and refrain from coming into close contact or sharing silverware with anyone who's sick.

And if you have to cough or sneeze, use your sleeve or a tissue -- but not your hands, the CDC says.

Summer surge?

The cases in the Unites States have been imported from international travel. Local outbreaks begin when the highly contagious illness spreads to those who are not immune to the virus due to a lack of vaccination. With the busy summer travel season just about to get underway there's concern about continued importation of measles among vacationers.

In his statement, Azar noted that the CDC has implemented an internal management structure to respond to the outbreaks, created toolkits to address vaccine hesitancy, and reached out to rabbinical and other associations with credible vaccine information.

"We cannot say this enough: Vaccines are a safe and highly effective public health tool that can prevent this disease and end the current outbreak," Azar said. "The measles vaccine is among the most-studied medical products we have and is given safely to millions of children and adults each year. I encourage all Americans to talk to your doctor about what vaccines are recommended to protect you, your family, and your community from measles and other vaccine-preventable diseases."