Take California, for example, where more than 90 people
have already been infected with measles this year. As in many states, parents in California don't have to vaccinate their children before kindergarten if they claim a religious or philosophical exemption.
Then there's Mississippi, which allows parents to opt out of vaccines only for medical reasons -- no other exceptions. That state has a 99.7% vaccination rate
-- and not a single case of measles this year.
Every state requires vaccinations, and every state also allows exemptions for medical reasons, such as if a child has a weakened immune system.
That's where the consensus ends.
In many states, parents have two other ways they can avoid vaccinating children: religious and philosophical reasons.
The vast majority of the country -- 48 states -- allows religious exemptions, according to the National Conference of State Legislatures
And 20 of those states also allow philosophical exemptions "for those who object to immunizations because of personal, moral or other beliefs."
The two states with the strictest vaccine requirements? Mississippi and West Virginia, which don't allow religious or philosophical exemptions.
The afflicted states
California, the epicenter of the current outbreak, allows exemptions for medical reasons and "personal beliefs." And parents have been using them.
During the last school year, 3.3% of California kindergartners -- about 18,200 -- were allowed to skip vaccinations, according to the Centers for Disease Control and Prevention
. The vast majority of exemptions were due to personal beliefs.
"Schools should maintain an up-to-date list of pupils with exemptions, so they can be excluded quickly if an outbreak occurs," the California Department of Public Health
But the number of measles cases in California over the past month -- 92 -- is higher than the median number of cases for the entire country for each year between 2001 and 2011, according to CDC figures.
Arizona is the next hardest-hit state, with at least seven measles cases already this year. Nearly 5% of Arizona kindergartners were able to skip vaccinations last school year due to medical reasons or, more commonly, their parents' personal beliefs.
New York and Utah each has at least three measles cases this year. New York allows religious exemptions, but not philosophical ones; Utah allows both.
Overall, about 94.7% of kindergartners across the country last year were vaccinated against measles, according to the CDC.
Mississippi and West Virginia, the two states that allow only medical exemptions to vaccination, have had no measles cases this year.
An incredibly contagious disease
Before the measles vaccine was introduced in the 1960s, many children came down with the disease by age 15. About 3 million to 4 million people
got measles each year in the United States. Among them, about 500 people a year died, and 4,000 developed encephalitis, or brain swelling.
Since then, the disease has largely disappeared in the United States. But international travel has spurred sporadic outbreaks in recent years.
Many of the recent measles victims are part of "a large, ongoing multi-state outbreak" linked to Disneyland in California
, the CDC said.
The disease is extremely contagious for several reasons:
• An infected person can spread it four days before developing a rash.
• 90% of people who are not immune and are close to someone with measles will also get infected.
• The virus is airborne.
• It can also live on infected surfaces for up to two hours.
Most, but not all, doctors agree
The overwhelming sentiment from the medical community is that the measles vaccine is safe and effective. But Arizona cardiologist Dr. Jack Wolfson is a rare voice of dissent.
"It's a very unfortunate thing that people die, but unfortunately people die," Wolfson said. "And I'm not going to put my child at risk to save another child."
Those words struck a nerve with Dr. Tim Jacks, a pediatrician whose own daughter has leukemia and, therefore, a weakened immune system.
"I can definitely, wholeheartedly say that the medical community, the medical literature does not support the statements he makes," Jacks told CNN's Anderson Cooper.
"The question I might have for him is, if you were in my situation, and your two children -- who you're doing your best to protect -- if they were suddenly exposed to measles, what would your thoughts be at that point?"