In the aftermath of what is being called “one of Africa’s worst flood disasters in living memory,” thousands in Sierra Leone risk infection in emergency shelters on the outskirts of the country’s capital where clean water and sanitation are hard to come by.

To take preventative measures, the West African country will vaccinate half a million people against cholera, an official announced three weeks after a devastating mudslide toppled Regent, killing some 500 people.

The situation quickly became a “public health emergency,” as aid workers said morgues were “overflowing” and corpses were lying out in the open heat.

Read More: A Mudslide in Sierra Leone Just Wiped Away Hundreds of Homes and People

“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 at the health ministry, told the Thomson Reuters Foundation.

Although generally not fatal, the water-borne bacteria can lead to rapid dehydration and kill within hours if left untreated.

Sierra Leone is no stranger to severe cholera outbreaks.

In 2012, the disease killed almost 400 people, and infected more than 25,000 others, the government said. Trying to avoid a relapse will be one of the country’s top priorities. But with a public health system already strained from one of the worst widespread outbreaks of Ebola, Sierra Leone will have to rely on international assistance.

500K people in #SierraLeone to gain access to emergency #cholera vaccine after devastating floods & landslides: https://t.co/6cn7XpACL6pic.twitter.com/9NaHdIqiAy

— WHO Sierra Leone (@WHOSierraLeone) September 5, 2017

Read More: WATCH: Better health for Sierra Leone starts with a single clinic

By Thursday, the country will receive 1 million doses of the oral vaccine from GAVI, the global vaccine alliance that is providing the medication.

Treatments will be administered over the course of three weeks, World Health Organization spokeswoman Laura Keenan wrote in an email to Thomson Reuters Foundation.  

“These life-saving 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.

Read More: People Caught in the World’s Worst Cholera Outbreak Won’t Be Getting Vaccines

Because the vaccine does not guarantee 100% protection, the government will also advise people to wash their hands thoroughly, boil water and prepare food hygienically, Thomas said.

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