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  'Cold Chain' is Key: Nicaraguan health officials transport the vaccine in an insulated box to maintain the 'cold chain,' a system by which the temperature is controlled in each stage from manufacturing to distribution. If the vaccine falls below thirty-five degrees or rises above forty-six degrees, it is no longer usable.

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  Vaccine touches down in Nicaragua: Flown directly from the manufacturing plant in Lansdale, Pennsylvania, the vaccine arrives by plane in Managua, Nicaragua. Two hundred miles from the clinic, it is still hours from reaching the children who need it.

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  A return to better health: In a rural clinic in Pantasma, Nicaragua, mothers await the arrival of the life-saving vaccine that will protect their children against rotavirus. Rotavirus is a preventable and treatable diarrheal disease that causes 500,000 deaths each year among infants and young children.

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  Traveling by truck: The journey continues by truck as the vaccine makes its way through Jinotega, a town 100 miles northwest of Managua, Nicaragua. It took 10 years of research and 16 years of development before scientists could make the vaccine available to children.

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  From truck bed to horseback: When the truck encounters a riverbed, the vaccine is transferred to horseback, its third method of transportation. In 2006, Nicaragua became the first developing nation to receive a vaccine the same year as the developed world.

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  A delicate balance: Horsemen Julio Mesa Zelodon and his son Santos balance the fragile cargo across the rocky terrain. The vaccine is often transported by horse, donkey or boat to reach the country's most remote areas.

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  End of the Road: After 2,000 total miles traveled, the blue box finally arrives at the clinic in Pantasma, Nicaragua. The box is taken inside, where the contents are quickly checked.

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  Still viable: A thermometer test confirms that the vaccine has stayed at a cool temperature and is safe to use.

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  A Mother Learns About the Vaccine: Inside the examination room, a mother learns about the vaccination process. The vaccine comes in a two milliliter dose. It requires three doses to protect her two-month-old daughter.

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  Taking steps to keep mother and child healthy: The journey is complete. A healthcare worker administers drops of the vaccine, as the little girl takes her first step in fighting the disease. Since the vaccine's introduction in 2006, Nicaragua has seen a 77 percent decrease in very severe rotavirus cases.

Gallery: The Journey of a Vaccine

Getting a lifesaving vaccine to the children who need it most

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Severe diarrhea, usually caused by rotavirus, is one of the leading causes of death for young children in poor countries. This is all the more tragic because vaccines for rotavirus exist – they just weren't reaching the kids who desperately needed them. But thanks to smart aid programs, that is changing. Follow this vaccines' journey from a lab in Philadelphia through the jungles of Nicaragua to reach the children who need it most. Each of them is Living Proof of what aid, when done properly, can accomplish.

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