Host Claudia Hammond of the BBC dives deep into some of the cases that could be causing parents to avoid vaccinating their children.
Across the world, fake news, a lack of access to reliable health care and medical supplies, poverty, religious leaders and a mistrust of authority are all cited as reasons for vaccination numbers falling.
So far this year, there have been more than a thousand cases of measles in the United States, spreading through communities where vaccination rates are low — from New York to the Amish in Ohio to the skeptical parents of Vashon Island in Washington state.
Health professionals think this is a frightening development for a preventable--and potentially life-threatening-- disease that was declared "eliminated" in 2000.
The mayor of New York has declared a health emergency, ordering mandatory vaccinations under threat of fines, but will such draconian measures be counter-productive?
Resistance to immunization is not new: A defective batch of a polio vaccine in the 1950s killed 10, paralyzed 200 and undermined confidence in all vaccines. This situation was echoed in a dengue vaccine scare in the Philippines which has led to more than 300 deaths from measles so far this year.
Conflict in the Ukraine and the Democratic Republic of Congo mean that infectious diseases like measles and Ebola are difficult to contain. But in other African countries like Ghana and Kenya, the arrival of a vaccine trial for malaria has been welcomed, so there is some good news.
Also, as Germany considers compulsory vaccinations because of a measles outbreak, we hear how it might be better to rely on advice from family and friends, text reminders and even ringing a gong, to persuade people to vaccinate.