The United States is experiencing its second-highest number of measles cases in nearly two decades. The Centers for Disease Control and Prevention (CDC) said the diagnosed cases have climbed to 465.

At least 19 states have reported cases of the highly contagious virus. The outbreaks are linked to people who traveled from countries such as Israel, Ukraine and the Philippines, where large measles outbreaks are occurring.

The disease is spreading in US communities that have relatively high numbers of people who have not been vaccinated against measles. Here are some answers to commonly asked questions about measles, which can cause serious complications among all age groups, especially young children.

How does measles spread?

Measles is one of the most contagious diseases on earth. It is a respiratory infection caused by a virus. The virus lives in the nose and throat of an infected person.

It can spread by direct contact with infectious droplets or through the air when an infected person breathes, coughs or sneezes. The measles virus can remain infectious in the air for up to two hours after an infected person leaves an area.

If other people breathe the contaminated air or touch the infected surface, then touch their eyes, noses or mouths, they can become infected.

Measles is so contagious that if one person has it, up to 90 percent of the people close to that person or who walk through the same area and are not immune also may become infected.

Infected people can spread measles to others from four days before through four days after the appearance of a rash.

That's why many health officials in places experiencing outbreaks, such as New York's Rockland County and New York City and Washington state, have declared public health emergencies.

Rockland County executives tried to ban unvaccinated minors from public places, including churches, schools and shopping centers. New York City just announced mandatory vaccinations in affected Zip codes.

Can antibiotics treat measles?

No. Antibiotics are used for infections caused by bacteria. Measles is caused by a virus. There is no specific treatment for measles. Health-care professionals try to prevent the disease by administering the measles, mumps, rubella (MMR) vaccine to children.

Nonimmunized people, including infants, may be given the measles vaccination within 72 hours of exposure to the virus to provide protection against the disease.

Pregnant women, infants and people with weakened immune systems who are exposed to the virus may receive a protein injection called immune serum globulin within six days of exposure to prevent measles or reduce the symptoms, according to the Mayo Clinic.

How dangerous is measles?

Measles can be deadly, especially for babies and young children. Some people may have severe complications, such as pneumonia (infection of the lungs) and encephalitis (swelling of the brain), which can lead to hospitalization and death.

Measles may cause pregnant woman to give birth prematurely or have a low-birth-weight baby. According to the CDC:

  • As many as one out of every 20 children with measles gets pneumonia, the most common cause of death from measles in young children.
  • About 1 child out of every 1,000 who get measles will develop encephalitis, which can lead to convulsions and can leave the child deaf or with intellectual disabilities.
  • For every 1,000 children who get measles, 1 or 2 will die from it.
  • Recent findings from researchers in California indicate a rare neurological complication that kills children years after they have been infected by the measles virus is more common than previously thought. The neurological disorder can lie dormant for years, and then is 100 percent fatal. There is no cure. For babies who get measles before being vaccinated, the rate is 1 in 609.

How effective is the measles vaccine?

The MMR vaccine is very effective. One dose of the vaccine is about 93 percent effective at preventing measles. Two doses are about 97 percent effective, according to the CDC.

The CDC recommends children get two MMR doses, starting with the first dose at 12 through 15 months of age and the second dose at 4 through 6 years of age.

If you were born before 1957, it is unlikely you need more MMR shots. That's because the CDC says almost everyone born before then has had measles, mumps and rubella and thus have natural immunity.

If you have no written documentation anywhere that you have had an MMR shot, talk with your doctor. In addition, if you are pregnant, clinicians say to wait to get the MMR shot until after the baby is born.

It is safe for women who are breast-feeding to get a measles shot, according to Wendy Sue Swanson, a Seattle pediatrician.

Why are infants at risk of measles?

If a baby's mother has had her MMR shots or had a measles infection in her life, she passed antibodies to her baby during fetal development and continued to pass them passively while breast-feeding.

Those antibodies provide protection for young infants and typically are thought to protect infants for up to six months or more. However, the length of protection is not known. Immunity wanes for these babies as they age and the mom's antibodies fail to persist.

Does the measles vaccine cause autism?

No, there is no link between autism and the MMR vaccine. This has been carefully examined by many doctors and scientists from around the world in large and thorough studies.

Scientists are studying what does make a child more likely to have autism, such as genes or an older father.

How did the idea that the MMR vaccine causes autism originate?

The debunked claim that there is a relationship between vaccines and autism largely stems from the late 1990s. At the time, autism diagnoses had been increasing, and doctors did not know why.

In 1998, British researcher Andrew Wakefield published a fraudulent paper, which was subsequently retracted, linking autism to the MMR vaccine.

Evidence emerged that Wakefield had been paid by attorneys for parents who were suing MMR manufacturers and that Wakefield's data were fraudulent. The Lancet retracted his study in February 2010.

That year, Wakefield was found guilty of professional misconduct by Britain's General Medical Council, and his license was revoked.

How are vaccines tested for safety?

Vaccines are among the most thoroughly tested medical products available in the United States. Before a vaccine can be considered for approval by the Food and Drug Administration, the manufacturer must show it is safe and effective through clinical trials.

This scientific process can take over a decade and cost millions of dollars. The FDA then examines these studies and determines whether a vaccine is safe, effective and ready to be licensed for use.

The FDA licenses only those vaccines that have data showing their benefits outweigh the potential risks. If there is any question about the data, or any holes in the data, the FDA will request further studies before approving the vaccine.

Vaccinate Your Family, the nation's largest nonprofit dedicated to advocating for vaccinations, provides a good explanation of the four systems in the United States that work together to help scientists monitor the safety of vaccines and identify any rare side effects that may not have been found in clinical trials.

For example, some side effects may happen only in 1 in 100,000 or 1 in 500,000 people.

Vaccine trials may not include certain populations, such as pregnant women or people with specific medical conditions who might have different types of side effects or who might have a higher risk of side effects than the volunteers who got the vaccine during clinical trials.

One system, known as the Vaccine Adverse Event Reporting System, or VAERS, relies on individuals to report vaccine reactions.

Anyone can report a reaction or injury, including health-care providers, patients and patients' representatives, such as caregivers or attorneys.

The system is co-managed by the FDA and the CDC. The purpose of VAERS is to see whether unexpected or unusual patterns emerge, potentially indicating a vaccine safety issue that needs to be researched further.

Another system, called the Vaccine Safety Datalink, is a collaboration between the CDC's Immunization Safety Office and eight large health-care organizations across the country.

It conducts studies based on questions or concerns raised from the medical literature and reports to VAERS. In addition, when new vaccines are recommended or if changes are made in how a vaccine is recommended, the VSD will monitor the safety of these vaccines.

The system has information on the kind of vaccine given to each patient, the date of vaccination and other vaccinations given on the same day. It also uses information on medical illnesses that have been diagnosed at doctors' offices, urgent-care visits, emergency department visits and hospital stays.

A third system, called the Clinical Immunization Safety Assessment Project, or CISA, is a national network of vaccine safety experts from the Immunization Safety Office, seven medical research centers and other partners.

It also addresses vaccine safety issues and conducts clinical research.

A fourth system, called the Post-Licensure Rapid Immunization Safety Monitoring, or PRISM, is a partnership between the FDA's Center for Biologics Evaluation and Research and leading health insurance companies.

It actively monitors and analyzes data from a representative subset of the general population.

Are religions opposed to vaccines?

No. Scholars believe no major religious group advocates against vaccinations on the basis of official doctrine, and experts say the majority support vaccinations.

Large majorities of Americans from all major religious groups say healthy children should be required to receive vaccinations to attend school, according to the Pew Research Center.

However, some individuals from various faith traditions believe vaccinating goes against their religious beliefs.

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This article was originally published by The Washington Post.