This week, Haiti was hit by Hurricane Matthew, a Category 4 storm that destroyed thousands of homes, flooded streets, and killed hundreds of people. In the aftermath of the storm, health officials and aid organizations are now worried about another threat — an uptick in the number of cholera cases in the Caribbean island.

An uptick in the number of cholera cases in Haiti

Haiti has had an ongoing cholera epidemic for six years now. The infectious disease was accidentally introduced by UN peacekeepers in 2010, months after an earthquake leveled much of the capital, Port-au-Prince, and killed as many as 300,000 people. Cholera, which causes severe diarrhea, is spread through contaminated water or food. More than 28,500 cases of cholera have already been reported so far in Haiti in 2016, according to the Pan American Health Organization (PAHO). And about 10,000 people have died since 2010.

Flooding caused by big storms helps spread water-borne illnesses like cholera, so Matthew’s appearance on the island is very bad news. The PAHO — and a bunch of local aid organizations, including Doctors Without Borders — are preparing for a possible surge in cases following the hurricane. Most places in Haiti don’t have proper sanitation, so heavy rains or storms flood open-air latrines, spreading infections. Any contact with the infected water — drinking, swimming, even just an open wound — can make a person sick.

Contact with infected water can make people sick

Hurricane Matthew has also destroyed several bridges, cutting off towns and making it hard for emergency responders to reach isolated communities. People in these towns may not be able to get access to clean drinking water, and if they get infected — or already have cholera — they won’t be able to access medical care. Flooded streets also make it hard for aid groups to move around the country and set up treatment centers with cots and IVs to rehydrate patients.

“Cholera is a highly transmissible disease through water contamination,” says Jean Luc Poncelet, PAHO/WHO Representative in Haiti. The south-western part of the island is “seriously devastated so the risk of transmission after a hurricane like that is increased.”

Over 4 million children in Haiti could be affected by Hurricane Matthew, the United Nations has said. "This is the worst storm Haiti has seen in decades and the damage will no doubt be significant," Marc Vincent, UNICEF's representative in Haiti, said in a statement on Tuesday. "Waterborne diseases are the first threat to children in similar situations — our first priority is to make sure children have enough safe water."

“Our first priority is to make sure children have enough safe water."

The PAHO and the World Health Organization deployed several field teams to Haiti to monitor the situation. Doctors Without Borders is also in the field and has a back-up cholera treatment center that can be up and running in a few hours, according to Tim Shenk, the senior press officer at Doctors Without Borders (MFS). “MSF teams are closely monitoring the situation and are ready to act if necessary, in coordination with other actors,” Shenk wrote in an email to The Verge.

The NGO Operation Blessing International has increased its production of chlorine, which can be used to disinfect surfaces and sanitize water for drinking, says Bill Horan, the NGO’s president. Their quarters in Port-au-Prince are currently producing 1,200 gallons of chlorine every day, which is then distributed to hospitals, schools, orphanages, and churches so that people can mop floors with it, clean counters and laundry, and kill cholera bacteria in drinking water.

“I think it’s going to be like a tsunami of disease.”

“There’s no doubt in anyone’s mind that there’s going to be a surge in the instances of cholera in Haiti post Matthew,” Horan says. “I think it’s going to be like a tsunami of disease, this cholera that is going to follow.”

Concern about other infectious diseases such as Zika and malaria is not as high right now in Haiti but could become a problem in four to eight weeks, says Poncelet at the PAHO/WHO. The country has reported nearly 3,000 Zika cases, according to the Centers for Disease Control and Prevention, although health officials believe that number could be much higher. And there were 17,094 reported cases of malaria in 2014. Immediately after a hurricane, there’s usually a decrease in the number of mosquito-borne diseases, Poncelet says. But the number goes up in the weeks afterwards, when floods recede but standing puddles of water remain where mosquitoes like to lay eggs.

Concerns over Zika aren’t as high right now

The main concern, for now, is cholera. And there’s only one way to solve the epidemic in the long term, experts say: invest in sanitation. Aid groups, donor countries, and charities should focus on building water systems and sewers instead of schools and orphanages, says Mark Merritt, a former FEMA official and the founder of DCMC Partners, a crisis management and public safety consulting firm. “Nobody likes to fund sewer plants and treatment facilities,” Merritt says. “Those are the things that usually aren’t built after a disaster because they’re not jazzy enough for someone to invest in them.”

It’s also key to not wait for the next natural disaster to build the necessary infrastructure so that cholera is wiped out from Haiti just like it was wiped out from Europe and the US. “We don’t know when the next spike is going to be but we need to be prepared for it,” says Jonathan Katz, a journalist and the former Associated Press chief correspondent in Haiti. “The best thing to do would be to eradicate the disease before the next spike occurs.”

Update and correction October 7th 9:15AM ET: The number of victims in Haiti has been updated. Mark Merritt’s title has been changed. He’s not the president of the disaster consulting firm James Lee Witt Associates, but the founder of DCMC Partners.