The World Health Organisation began a campaign on Saturday to prevent outbreaks of cholera in temporary camps in South Sudan housing thousands of people who have fled the country's brutal two-month-old conflict.
The first phase will see around 94,000 people vaccinated against the disease in Minkaman camp in Awerial county, followed by 43,000 in camps around the capital Juba.
"Although currently there is not a cholera outbreak, people displaced by the recent conflict and living in the camps are at risk due to poor sanitary conditions and overcrowding," the WHO said in a statement.
The programme is being carried out in coordination with the South Sudanese government, with the help of the Red Cross, Doctors Without Borders (MSF) and UNICEF.
"Minkaman camp and Juba camp have been selected because of the relative stability of the situation and easier access in those places," said Dr Abdinasir Abubakar of the WHO's disease surveillance and response team.
"We are also looking at other camps, and once the accessibility and security improves, we will expand the cholera vaccination campaigns into these areas."
South Sudan has been embroiled in a bloody conflict since December 15, 2013 pitting troops loyal to President Salva Kiir against rebels linked to his sacked vice president Riek Machar.
The unrest in the world's newest nation has killed thousands of people and displaced close to 900,000, including tens of thousands who have crammed into UN bases in fear of ethnic attacks by either Kiir's Dinka tribe or Machar's Nuer.
Cholera is an acute diarrhoeal infection caused by eating contaminated food or water, with children facing a particularly high risk of infection. It can kill in a matter of hours due to rapid dehydration.
The disease, which often breaks out around natural disasters or conflicts, affects between three million and five million people per year, with up to 120,000 dying from the disease.