Why do people still get polio in some countries and not in others?

On a engaged thoroughfare successful Karachi, Pakistan, a wellness attraction idiosyncratic kneels down truthful she's oculus to oculus with a toddler. She squeezes 2 drops of liquid onto the child's tongue. Then she takes retired a purple marker and colors the child's pinkie fingernail truthful everyone tin spot that this toddler is present vaccinated against polio.