(CNN) -- A cholera outbreak has erupted this summer in Africa, killing more than 600 people in the neighboring countries of Nigeria and Cameroon.
Nigeria's Health Ministry said more than 350 people have died since June and the infection threatens to spread to the entire country, the most populous in Africa. In neighboring Cameroon, nearly 300 people have been killed.
"There's a lot of people crossing over the border all the time," said Dr. Eric Mintz, the leader of the Centers for Disease Control and Prevention's global water sanitation and hygiene epidemiology team. "So it's not surprising that the cholera is also crossing over the border."
Nigeria's outbreak had sickened more than 6,400 people and killed 352 people by Wednesday, the federal Ministry of Health reported.
"Although most of the outbreaks occurred in the northwest and northeast zones, epidemiological evidence indicates that the entire country is at risk," the ministry reported.
Cholera occurs in much of the country under normal conditions, but the lack of clean drinking water and recent flooding following heavy rains are fueling the spread of disease, the ministry reported.
Two-thirds of rural Nigerians lack access to safe drinking water, and fewer than 40 percent of the people in the affected states have access "to toilet facilities of any description," the health ministry said.
The intestinal infection causes diarrhea and vomiting that can cause severe dehydration without prompt attention, according to the World Health Organization.
About 120,000 people die worldwide each year from cholera, and the disease can kill within hours if untreated, the World Health Organization says.
Cholera is spread through water contaminated with human waste.
"What puts people at risk is a lack of safe drinking water," Mintz said.
Boiling the water or treating it with chlorine or filtration or other ways to remove the contaminants can make it safe.
Untreated, cholera can kill within hours, WHO said.
Mintz said the Cameroon outbreak began in May and has exceeded 3,000 cases.
Under ideal conditions, cholera's fatality rate ought to be less than 1 percent, he said. But Cameroon's conditions are not ideal. Many people live far from the medical aid that could help them rehydrate and replace the sodium and
potassium lost in their stools and vomit.
"That delay can be critical," Mintz said. "If you don't get the fluids and the electrolytes replaced through hydration, you can go into shock and die as a result."
The country's northern region has been particularly hard hit. Three of 10 prisoners diagnosed with cholera at Maroua Central Prison in the far north were among the dead, the officials said.
Mintz said the rugged and remote region has been affected repeatedly by cholera epidemics, most recently last year.
About 70 percent of people living in the country's far north, bordering Nigeria and Chad and the Central African Republic, do not have access to potable water, according to a Ministry of Water and Energy official.