Most researchers currently believe that United Nations peacekeeping soldiers introduced cholera to Haiti in October of 2010.
After all, Haiti hadn't recorded cholera for as long as a century, Nepal had experienced a cholera epidemic in the months preceding the soldiers' arrival, and the Haitian and Nepalese cholera strains were found to be nearly identical.
But it's not that simple, says a research group based at the University of Maryland.
These researchers have found two very different cholera strains in some of the first Haitians to be struck by the disease.
One is a so-called 01 serotype with close resemblance to the Nepalese strain, found in about half the patients sampled. The other is a type called non-01/O139 that has never been known to cause an epidemic; it was found in 21 percent of patients. Another 7 percent of patients harbored both types.
What can this mean? It's not clear, but the leader of the team, Rita Colwell, thinks cholera germs have been lurking undetected in the Haitian environment for a long time.
"This suggests that it's very likely that local (Haitian) strains are involved," Colwell told Shots. "Because no one has tested for pathogenic cholera strains in that country before, we have no evidence that it wasn't there already."
I asked Colwell if she thinks one strain was introduced by the Nepalese soldiers and the other was native to Haiti, or at least predated the current epidemic.
"The introduction (from Nepal) can't be ruled out but it can't be proven either," she replied. "I think the evidence is at best circumstantial, and it is not sufficient to account for the entire epidemic."
Colwell, an internationally recognized expert on the interaction of cholera and environmental factors, thinks Haiti's explosive epidemic is most likely explained by the "perfect storm" of three converging factors.
"You have this massive earthquake in January 2010," she says. "The geology of Haiti is limestone. With earthquake effects disrupting the rivers, the rivers become very alkaline." Colwell's studies have shown that the bacterium Vibrio cholera thrives in alkaline waters.
"Then Haiti had one of the hottest summers on record," she continues. That warmed the estuaries where cholera likes to breed in tiny crustaceans, further abetting the bacteria's cause.
"That was followed by a hurricane that skirted Haiti, causing heavy rain and flooding," Colwell points out. "With all the river systems churned up with nutrients and warm water, and proper alkalinity, it would be ideal for the organism to become quite dominant."
No doubt some will be skeptical. With Haiti's climate, geography, proneness to hurricane-related flooding and notorious poor sanitation, some may doubt that cholera epidemics could have been absent, or overlooked, until 20 months ago.
But some scientists think Colwell is onto something.
Dr. David Sack of the Johns Hopkins Bloomberg School of Public Health doesn't buy the hypothesis that Nepalese peacekeepers started the epidemic by contaminating a leaky latrine upstream from the first cholera cases.
He thinks the epidemic exploded too soon after the Nepalese reportedly arrived in Haiti. UN officials tell him that was on October 8, and the first cholera case was recorded on October 12 in a town near the UN camp.
"Cholera's incubation period is at least 24 hours, sometimes two or three days," Sack told Shots. "Just to have a cholera vibrio floating downstream, and considering the dilution factor – well, it raises questions in my mind. Not that it wasn't imported. I think it was imported. I just question when it was imported."
Sack thinks we may never know how the South Asian strain of cholera got into Haiti.
As for the non-01/O139 strain, Sack thinks it might have been hiding in Haiti's environment, waiting for Colwell's perfect storm.
In any case, Sack says the Colwell group has focused new attention on the environmental factors behind the epidemic. And both agree that, going forward, controlling cholera in Haiti will take a massive effort to provide clean drinking water plus vaccination against the disease.
"I would be very strongly in favor of a combination of provision of safe water and vaccine," Colwell says. "That could be very, very effective."
A pilot project this spring has vaccinated close to 100,000 Haitians in Port-au-Prince and the river delta where cholera first appeared in large numbers. But it will take millions of doses, renewed every two to three years, to protect most Haitians at risk. Sack wonders where that much vaccine will come from; it's multiples of the world's current supply.
So far, Haiti has counted 557,397 cases of cholera since the epidemic's beginning, and 7,278 deaths. The current rainy season that began in March, which fosters cholera outbreaks, has added nearly 30,000 cases and 250 deaths to that total.
Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.