YAOUNDE (Thomson Reuters Foundation) - Sierra Leone will vaccinate half a million people against cholera as thousands risk infection in emergency shelters with limited access to clean water and sanitation, an official said three weeks after one of Africa’s worst mudslides.

An estimated 500 people were killed when a mountain collapsed on the outskirts of Freetown, capital of the West African country, which is recovering from the Ebola crisis.

“If we have a case of cholera that is spread into those communities, because of the endemic situation of the sanitation, it will spread like wildfire,” Harold Thomas, an officer with the health ministry, told the Thomson Reuters Foundation.

The water-borne bacteria is generally not fatal, but it can kill in just a few hours when diarrhea and vomiting cause dehydration, especially among the elderly.

Sierra Leone’s worst recorded cholera outbreak in 2012 killed almost 400 people and infected more than 25,000 others, the government said.

More than one million doses of the oral vaccine are set to arrive in Sierra Leone on Thursday and will be administered over three weeks, World Health Organization spokeswoman (WHO) Laura Keenan said via email.

Sierra Leone’s public health system is “stretched” after the world’s worst Ebola epidemic, the GAVI global vaccine alliance, which is providing the drugs, said in a statement.

“These lifesaving vaccines ... have the potential to prevent a cholera outbreak before it has the chance to bring more misery to a country that has already suffered enough,” GAVI’s chief executive Seth Berkley said.

Ebola killed more than 11,000 people across Guinea, Liberia and Sierra Leone between 2013 and 2016.

As the cholera vaccine does not provide 100 percent protection, the government is also advising people to wash their hands, boil water and prepare food hygienically, Thomas said.