(CNN) -- More people have been infected with measles in the United States during the first four months of this year than have been infected in the first four months of the past 18 years, according to the Centers for Disease Control and Prevention.
Data released Thursday reveal a dramatic rise in measles outbreaks. Health officials say 13 outbreaks and 129 cases have been recorded this year, according to the CDC's Morbidity and Mortality Weekly Report.
California is reporting the highest numbers, with 58 cases from January 1 to April 14. That's the most cases of measles in the state since 1995. In the past 13 years, the average number of measles cases was nine per year.
The CDC says many of the California cases are a result of people visiting the Philippines, which is experiencing a very large measles outbreak; at least 20,000 confirmed and suspected cases have been reported in the Asian nation. Visitors may pick up the disease and bring it back to the U.S., potentially infecting those who cannot be vaccinated against the measles because they are too young, for example, or who have intentionally remained unvaccinated.
Another problem that may be contributing to the increase in cases is a lack of awareness among doctors.
"Because of the success of the measles vaccine, many clinicians have never seen measles and may not be able to recognize its features," Dr. Julia Sammons writes in a commentary published Thursday in Annals of Internal Medicine.
Measles is one of the most contagious infectious diseases. Symptoms usually include fever, cough and conjunctivitis, along with a rash. In rare cases, measles can lead to pneumonia and brain infections, which can be fatal.
The last time there was a reported death from measles in the U.S. was in 2005, says Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases. Schuchat credits the CDC's Vaccines for Children program with contributing to high immunization rates and the dramatic decrease of measles in the U.S.
In the U.S., half of all children under 19 are eligible for the program, which provides vaccines to children whose parents or caregivers can't afford them.
According to the CDC, vaccinations have proved to be very successful in the U.S., preventing an estimated 323 million illnesses and 732,000 deaths. Schuchat says it's impossible to predict what the coming months will bring, but she thinks more people could die from measles.
"Unfortunately, it's probably just a matter a time."
The CDC recommends that children get two doses of the measles vaccine beginning at 12 months, with the second dose between the ages of 4 and 6 years old. Infants 6 to 11 months old should get the vaccine before international travel.
Schuchat says it's absolutely crucial for measles vaccinations to be up to date.
To further reduce the spread of measles, the CDC also recommends that all U.S. residents born after 1956 ensure that they are still immune to measles or receive a new MMR vaccination, particularly if they expect to travel outside North or South America.