United States health officials say they're seeing an increase in the number of parents refusing to vaccinate their children.
Several states have experienced large outbreaks of vaccine-preventable illnesses recently. Colorado this year experienced its worst outbreak of pertussis, commonly known as whooping cough, in two decades.
Dr. Rachel Herlihy of the Colorado Department of Public Health and Environment says vaccine refusal is a dangerous trend: "You can have a playgroup where four or five children in that playgroup perhaps aren't vaccinated. And that certainly is a situation where there is a much greater risk of a transmission."
Some doctors worry that long-gone diseases could one day reappear, but a small minority believe vaccines do more harm than good.
Dr Patricia Kloor, a chiropractic pediatrician who runs a practice in Westminster, often works with parents who are worried about the impact vaccines may have on their children's health. Kloor says the parents are a fearful of the contents of vaccines, which she says often include aluminum and dangerous chemicals.
Kloor says she helps parents find alternatives to vaccines, including eating organic foods, living a healthier lifestyle and getting chiropractic treatments.
However, Dr. Joseph Marceny, a pediatrician, said research linking vaccines to autism has been debunked, and that vaccines have become safer, generally.
"I've seen some tragic, tragic illnesses that are preventable", says Marceny, "so I think it's real important to be careful about saying that no vaccines are the way to go."
Immunizations given to children two-and-under have led to dramatic declines in vaccine-preventable diseases in the U.S., but vaccine refusal rates for kindergarteners top 6 percent in some states.
The U.S. government blames recent outbreaks of illnesses like whooping cough, chickenpox and measles on an increase in vaccine refusal during the last several years.