NEW YORK — They’re considered one of mankind’s greatest medical achievements, yet people have balked at vaccines almost since the time of the first vaccination — in 1796, when an English country doctor named Edward Jenner inoculated an 8-year-old boy against smallpox.

In the mid-1800s, people protested in the streets of Victorian England after the British government began requiring citizens to get the vaccination. Many opponents mistrusted doctors and were wary of a medical treatment they didn’t understand. In the early days, the closely related cowpox virus was used to immunize people against smallpox.

“People were afraid that if you got the cowpox vaccine you would turn into a cow,” said Dr. Paul Offit, a vaccine researcher at Children’s Hospital of Philadelphia who is an outspoken critic of anti-vaccination groups.

More than a century and a half later, there’s still an undercurrent of vaccine dislike and distrust in the United States as illustrated by the measles outbreak that started in December at Disneyland — likely brought in from overseas as has been the case in recent years. Many of those who got and spread the highly contagious illness hadn’t gotten the childhood shots.

All this despite medical science’s proven successes in wiping out not only the much-feared smallpox and polio, but nearly eliminating other serious illnesses like diphtheria, German measles, lockjaw and mumps in the United States. Through it all, anti-vaccine sentiments have ebbed and flowed.

“It is fair to say that for as long as we’ve had vaccines, governments have worked to promote those vaccines — and segments of the population have resisted,” said Jason Schwartz, a medical historian who studies vaccine policy at Princeton University.

In the U.S., opposition to vaccines was most intense in the late 1800s and early 1900s, said Susan Lederer, chair of medical history and bioethics at the University of Wisconsin.

That’s when organizations like the Anti-Vaccination League of America and the American Medical Liberty League led the charge. Some used photos depicting vaccinated children with scars and missing limbs and eyes, claiming immunizations were to blame.

Vaccines sold at the time were unregulated, and many were not only ineffective but also risky because of how they were made or given. But opposition faded, and legal challenges to school-attendance vaccination laws were shot down in court.