The number of people infected with measles linked to the outbreak at Disney amusement parks in southern California now stands at 78, according to health officials.
The overwhelming majority of cases - 68 - have been in California. The other 10 have been reported in six other states and Mexico.
Most of those people hadn't gotten the measles-mumps-rubella, or MMR, vaccine, the Associated Press reported.
Health officials said that about one of every four California patients had to be hospitalised, the Los Angeles Times reported.
Public health officials are urging people who haven't been vaccinated against measles to avoid the Disney parks where the outbreak originated. California state epidemiologist Gil Chavez also urged the unvaccinated to avoid places with lots of international travelers, such as airports.
Read: Measles outbreak linked to Disney theme parks
"Patient zero" - or the source of the initial infections - was probably either a resident of a country where measles is widespread or a Californian who traveled abroad and brought the virus back to the United States, the AP reported.
The outbreak is occurring 15 years after measles was declared eliminated in the United States. But the new outbreak illustrates how quickly a resurgence of the disease can occur.
And health experts explain the California outbreak simply.
"This outbreak is occurring because a critical number of people are choosing not to vaccinate their children," said Dr. Paul Offit, director of the Vaccine Education Center and an attending physician at The Children's Hospital of Philadelphia's Division of Infectious Diseases.
Read: Could your kid get measles?
"Parents are not scared of the disease" because they've never seen it, Offit said. "And, to a lesser extent, they have these unfounded concerns about vaccines. But the big reason is they don't fear the disease."
On Friday, the American Academy of Pediatrics (AAP) recommended that all parents vaccinate their children against measles.
"Vaccines are one of the most important ways parents can protect their children from very real diseases that exist in our world," Dr. Errol Alden, the academy's executive director and CEO, said in a news release. "The measles vaccine is safe and effective."
Dr. Yvonne Maldonado, vice chair of the academy's Committee on Infectious Diseases, said: "Delaying vaccination leaves children vulnerable to measles when it is most dangerous to their development, and it also affects the entire community.
"We see measles spreading most rapidly in communities with higher rates of delayed or missed vaccinations. Declining vaccination for your child puts other children at risk, including infants who are too young to be vaccinated, and children who are especially vulnerable due to certain medications they're taking."
Read: Vaccines – serious side effects quite rare
The United States declared measles eliminated from the country in 2000. This meant the disease was no longer native to the United States. The country was able to eliminate measles because of effective vaccination programs and a strong public health system for detecting and responding to measles cases and outbreaks, according to the US Centers for Disease Control and Prevention.
But in the intervening years, a small but growing number of parents have chosen not to have their children vaccinated, due largely to what infectious-disease experts call mistaken fears about childhood vaccines.
Researchers have found that past outbreaks of vaccine-preventable diseases are more likely in places where there are clusters of parents who refuse to have their children vaccinated, said Saad Omer, an associate professor of global health, epidemiology and pediatrics at Emory University School of Public Health and Emory Vaccine Center, in Atlanta.
These so-called "vaccine refusals" refer to exemptions to school immunization requirements that parents can obtain on the basis of their personal or religious beliefs.
Read: Delaying Measles vaccines may raise seizure risk: study
"California is one of the states with some of the highest rates in the country in terms of exemptions, and also there's a substantial clustering of refusals there," Omer said. "Perceptions regarding vaccine safety have a slightly higher contribution to vaccine refusal, but they are not the only reason parents don't vaccinate."
Other reasons include the belief that their children will not catch the disease, the disease is not very severe and the vaccine is not effective, Omer noted.
A big contributing factor to the parents' continuing concerns about vaccine safety was a 1998 fraudulent paper published and later retracted in the medical journal The Lancet. The study falsely suggested a link between the measles-mumps-rubella vaccine and autism. The lead author of that paper, Andrew Wakefield, has since lost his medical license for having falsified his data.
Several dozen studies and a report from the Institute of Medicine have since found no link between autism and any vaccines, including the MMR vaccine.
Read: Measles outbreak hits vulnerable Britain
Researchers say that those who refuse vaccines tend to share similarities.
"In general, they're upper-middle to upper class, well-educated - often graduate school-educated - and in jobs in which they exercise some level of control," Offit said. "They believe that they can Google the word vaccine and know as much, if not more, as anyone who's giving them advice."
Omer added that recent data has shown that measles cases tend to disproportionately involve people who are not vaccinated. "The higher the vaccination rates, the lower the frequency and size of outbreaks," he noted.
The American Academy of Pediatrics, the US Centers for Disease Control and Prevention and the American Academy of Family Physicians all recommend that children receive the MMR vaccine at age 12 to 15 months, and again at 4 to 6 years.
Read: Measles surges in UK years after vaccine scare
The most common side effects of the MMR vaccine are a fever and occasionally a mild rash. Some children may experience seizures from the fever, but experts say these seizures have no long-term negative effects.
The majority of recent outbreaks have been traced back to unvaccinated US residents. Last year, 644 measles cases were reported to the CDC, the highest number of cases recorded since the disease was declared eliminated.
Measles is one of the most contagious of human diseases. The airborne virus can linger in an area up to two hours after an infected person leaves, and approximately 90 percent of people without immunity will become sick if exposed to the virus.
Serious complications from measles can include pneumonia and encephalitis, which can lead to long-term deafness or brain damage. An estimated one in 5 000 cases will result in death, according to Offit.
"If a child died of measles in southern California, I think people would start vaccinating," Offit said. "I think it will take more suffering and more hospitalisations and more deaths to not see these outbreaks. We're compelled by fear, and we don't fear this disease enough."Also read:
Measles cases at a 20-year high
Vaccines – serious side effects quite rare
Childhood brain tumours vulnerable to measles vaccine