"Go see the child," Samir Saha said.

It was a little after dawn in Dhaka, the capital of Bangladesh, and Saha was sending me to go see a girl whose life had been permanently altered by a preventable disease. The situation would show me the importance of vaccines.

Saha is a microbiologist, internationally renowned for his research on a bacterium called pneumococcus. These bacteria are found everywhere in the modern world. They are easily spreadable through sneezing or casual contact and can live in the nasal passages of people with healthy immune systems without harming them.

However, when our defenses fail us, pneumococcus can migrate, multiply and set off life-threatening diseases, including pneumonia, which is an infection in the lungs. It leads to rapid breathing, fever, a rattling cough and blue lips. Pneumococcus can also cause dangerous infections in the bloodstream, as well as meningitis, which is an infection in the tissue lining of the brain.

Vaccine-Preventable Diseases Cause Much Suffering

Young children are especially vulnerable to pneumococcal bacteria. Young children in places without access to antibiotics and good medical care are the most vulnerable of all. At the start of the 21st century, pneumococcal disease was killing more than 800,000 children worldwide every year, including 750,000 infants and children younger than the age of five. The majority of those deaths occurred in impoverished countries such as Bangladesh.

In 2015, a vaccine that prevents pneumococcal disease in children, called pneumococcal conjugate vaccine (PCV), reached Bangladesh. Saha's research team is tracking its progress. If PCVs are effective around the world, it could save thousands of young lives.

Coming from a country like the United States, it can be easy to imagine that the most pressing vaccine challenge is convincing skeptical parents that they should vaccinate their children. Those efforts are important but even more urgent is the international collaboration to get new vaccines to children in the developing world. To this day, vaccine-preventable diseases cause suffering for many people in countries like Bangladesh.

This is why Saha sent me to see 11-year-old Sanjida Sahajahan. I hopped into a van that bumped through crowded Dhaka streets to her family's home.

Every Childhood Vaccination But One

When I arrived, Sanjida was propped up in a small plastic armchair beside the family bed. When she was three years old, she developed meningitis, which can cause irreversible damage to the membranes that surround the brain and spinal cord. To this day, she has no control over her head, her facial expressions or the sounds she makes. She is unable to form words.

Her mother, Nazma, explained that the illness had started as an unexplained fever, and a few days later, Sanjida was having convulsions or uncontrollable muscle contractions. Her parents brought her to the hospital, but by the time doctors saw her, she was losing consciousness.

As a baby, Sanjida received every vaccine in Bangladesh's national immunization plan at the time, including whooping cough, measles, tetanus, hepatitis B and polio. In a way, her vaccination history is a summary of major advances in global health. However, at the time, she did not receive the vaccine against pneumococcal infection. Even though it was already being injected into children across the United States, places like Bangladesh, which needed the vaccine desperately, could not afford it.

No one can accurately tally the total number of lives saved by widespread vaccination, but it remains one of the greatest achievements of modern medicine. Measles, for example, killed more than two million children a year worldwide in the 1980s. By 2015, vaccination had dropped the death toll to 134,200, according to the World Health Organization. Mass vaccination ended polio in all but three countries. Bangladesh and its neighbor India were pronounced polio-free in March 2014.

Pharmaceutical Companies Charge High Prices

Delivering vaccines in Bangladesh isn't easy, but the Bangladeshi inoculation program is widely respected for its remarkable reach. Vaccines must be kept at just the right cold temperature to preserve their potency. Maintaining this "cold chain" is difficult in countries like Bangladesh, which has a hot climate and shaky power grids. Yet, Bangladesh has worked hard to preserve the chain. Bangladesh has also worked hard to spread public service ads about inoculation, and the general public remains open-minded about vaccines.

The real problem is that people often cannot pay what vaccine manufacturers charge, even though they desperately need the vaccine. Vaccines are usually made by private companies that aim to make money. Until recently, vaccine manufacturing has been dominated by a few U.S. and European giant pharmaceutical companies.

Developing a new vaccine is expensive, so manufacturers set vaccine prices high to make up the cost. It took decades to develop the pneumococcal vaccine for children, partly because there are nearly 100 versions, or serotypes, of pneumococcal cells. Serotypes that make people sick in one part of the world might not make people sick in another part of the world. Serotype 1, for example, causes little disease in the United States but is a prime source of pneumococcal illness and death in Africa and South Asia.

The first children's pneumococcal vaccine was one of the most expensive in history. Launched in 2000, it was designed to work against the seven serotypes responsible for most of the disease in the United States. However, it did not work against serotype 1, the main cause of pneumococcal illness in Africa and South Asia. The vaccine costs $232 per four-dose course, which most people in developing countries could not afford.

The situation was unfair, according to Orin Levine, vaccine delivery director at the Bill and Melinda Gates Foundation. "The chances of a kid dying of pneumococcal disease in the rich world were a hundredfold less," he says.

Health Officials Are Optimistic

Others who shared Levine's opinion started the Global Alliance for Vaccines and Immunisation, or Gavi. The alliance channels wealthy nations' resources, private funds and government aid from countries such as the United States, the United Kingdom and Norway into vaccine support for poorer countries. It helps reduce the cost of vaccines for developing countries.

Gavi devotes a half-billion dollars a year to PCV support. The alliance has also made a special arrangement with PCV manufacturers. Thanks to these arrangements, the manufacturers have developed new versions of the vaccine that work against serotype 1 and other serotypes common in Africa and Asia.

Since March 2015, Bangladeshi health officials have received deliveries of discounted PCV every three months. The vaccines, health officials say, are reaching families all over the country and Saha's team has noticed a drop in the number of pneumococcal disease cases.

Saha hopes that one day, the vaccines will wipe out pneumococcal disease completely, and his research will no longer be needed.