In 2015 in the U.S. and Canada — and now again in 2019 — an outbreak of measles struck, setting loose what seems like an endless discussion about immunization. On one side are public health professionals, urging people to check their immunization status. On the other are impassioned individuals and organized anti-immunization crusaders.In 2015, I heard a discussion on the CBC pitting health officials against a woman who claimed she had done her research, showing that serious adverse events were common in routine immunization. What she called research was actually chats she had had with other people who oppose immunization. She also brought up as “evidence” the long-discredited research that falsely claimed a link between measles and autism.When the interviewer pointed out that her choice not to immunize her children reduced herd immunity and thereby put others, including immuno-compromised children and adults, at risk of preventable diseases, she replied that she was sorry about that. When the interviewer pointed out that the source of recent outbreaks were religious communities and other groups that opposed immunization, she replied that they had a right to their beliefs and that it was about “freedom of choice.”Why the debate? Immunization campaigns have been so successful that the public no longer has personal experience with the diseases that have been prevented, including diphtheria, whooping cough, tetanus, polio and certain types of meningitis and ear infections. In the very recent past, it was usual for the public to have had direct experience with these diseases. The public knew of U.S. president Franklin Delano Roosevelt’s post-polio, much-diminished state. It is remarkable to think that FDR was so much weakened from the disease that if he had died a little earlier, the Second World War might have been lost.I had all the classical childhood diseases. I still remember spending a week in a dark room with measles. I recall high fever, nightmares and what must have been delirium, severe light sensitivity and a seizure. Lacking experience with measles, many people think of measles as nothing more than a fever and rash. In fact, it is a severe respiratory disease, including bronchopneumonia, ear infections, light sensitivity, with invasion of the brain often severe enough to cause encephalitis and even permanent brain damage.Before the measles, mumps and rubella vaccine was approved in 1963, almost everyone got measles by age 18. In Canada, there were 300,000 cases every year. After the vaccine, that number fell to less than 20. In recent years there have been a few hundred cases after travellers brought the virus back to Canada. In unimmunized populations, one in five children are hospitalized with measles, and one in 1,000 die.As a Stanford medical student in 1963, I received an opportunity to work in southern Mexico in a small village called Aquacatenango, a pueblo of about 1,000 people who spoke a Mayan dialect called Tzeltal. Just prior to my arrival, a major measles epidemic hit the town, with devastating consequences for the local children. There were many fatalities and some children were left with hearing loss, chronic chest and ear infections and brain damage from encephalitis.The day after I arrived, a distraught father asked me to see his four-year-old daughter. Completely normal before getting measles, she was curled up in a corner of the house, unresponsive, in a deep coma that had lasted for weeks. Sadly, I had to tell the father there was nothing that I could do for her.A week later, I was surprised to be invited by the father to attend a curing ceremony. He felt obligated to try one last thing. The ceremony was to be conducted by a curandero (a native healer), one I was told was a difficult and dangerous person. In Mayan culture, most serious diseases were thought to arise from “loss of soul.” Over-simplifying, that was thought to occur from a fright or from witchcraft. Curing ceremonies were organized to recover the soul.The ceremony began about 11 p.m. in a night well-lit by a nearly full moon. We walked around the town and surrounding cornfields. About 4 a.m., we found the bleached skull of a cow. The curandero announced that this was the place where the child was frightened and lost her soul. We returned to the home, where the child was lying on a bed of straw. At one end was a Catholic cross and a lit candle. At the other end was a chicken tied to a post. Palm fronds were arranged over the child in a tent-like structure.We began to circle the child, singing in Tzeltal and simple Spanish. After about 15 minutes, the curandero turned to me and asked me for my watch, which he said was required for the completion of the ceremony. I refused to give him the watch. He became enraged, kicked over the assembly, liberating the chicken, which flapped wildly around the house, and stated that the child would die and it was my fault.He loudly announced that I would suffer for my action. He refused payment for the ceremony and stormed out of the house. At this point, the father stated that this was bad behaviour by the healer and that I was not responsible if the child died. He insisted that we share a meal. The child died several days later. The curandero left town and did not return during my sojourn in Aquacatenango.The very success of immunizations have allowed those who deny the efficacy of immunizations, or who have issues with modern medicine, to find a receptive audience. These deniers distrust doctors and science, sometimes with good reason. They allow their own personal “freedom” to do what they please with their bodies to trump the public good. Important vaccine side effects do occur, but their frequency is many orders of magnitude less than the frequency of major complications and death from the diseases they prevent.Unless we reach those who distrust modern approaches to the prevention of disease, it is clear that the gains of the past are in danger of being lost and diseases that were eliminated are at risk of reappearing. The earth is not flat. The climate is warming. Vaccines work and have eliminated diseases that we hope never to see again.Dr. Michael Klein is an emeritus professor of family practice and paediatrics and a senior scientist emeritus at the University of B.C. He is the author of Dissident Doctor: Catching babies and challenging the medical status quo.Letters to the editor should be sent to firstname.lastname@example.org. 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