An outbreak of hemorrhagic fever syndrome has spread across a northern region of South Sudan. Since the start of the low-intensity outbreak in late December 2015, 51 cases, including 10 deaths, have been recorded.

As of now, the World Health Organization (WHO) does not recommend any restriction of travel and trade to the affected area. But the underlying cause of the outbreak is still unknown.

The symptoms of the disease are similar to that of Ebola: unexplained bleeding, fever, fatigue, headache and vomiting. Like most hemorrhagic fevers, the disease can be fatal in severe cases that lead to shock and organ failure.

However, unlike Ebola, there is also no evidence that the disease in this outbreak is spread from person to person. The symptoms are also less severe, the WHO said in a statement, and rapidly resolve with treatment.

Samples from 33 patients were also tested for Ebola, and all came back negative. The samples also tested negative for Crimean-Congo hemorrhagic fever, Marburg virus disease, Rift Valley fever, yellow fever, West Nile virus and the Zika virus.

Five samples did, however, test positive for O’nyong-nyong virus, three were positive for Chikungunya, and one sample tested positive for dengue virus – all three of which are transmitted by mosquitoes.

Because of this, some say the mysterious disease might be an undiagnosed mosquito or tick-borne viral illness. But none of these viruses cause bleeding as seen in the current outbreak or explain the high mortality rate.

WHO medical specialist and spokesman Eric Bertherat said the available data points to a different type of disease.

“The fact that three-quarters of the cases are below [age]20 and that 87.5 percent of the affected adults are women, is not in favor of a mosquitoes-transmitted disease but rather to an event linked to a specific behavior or to the environment,” Bertherat wrote in an email to Humanosphere.

“The concentration of population (and predominantly women and children) and domestic animals around patches of stagnant water can evoke a water-transmitted disease such as leptospirosis.” he wrote. “But we cannot discard a toxi-infection by a mycotoxin (aflatoxin for instance).”

Leptospirosis is a bacterial disease spread through the urine of infected animals, and aflatoxins are poisons produced by certain molds. Both are plausible explanations for a region lacking adequate sanitation resources: More than 30 percent of the population lacks access to safe water supplies, according to UNICEF, and a mere 15 percent has access to adequate sanitary latrines.

While scientists work to determine the cause of the recent outbreak, the current reality is that there is a lot we don’t know. The good news is that no deaths have been reported since Feb. 28, which, according to Bertherat, is a clear indication that antibiotics and supportive therapies have been effective in the more recent cases of the disease.

The bad news is that the outbreak has occurred close to Darfur in Sudan, where at least 469 cases of undiagnosed viral hemorrhagic fever, including 120 deaths, were reported between August and November 2015. Because of frequent cross-border movement between Sudan and South Sudan, the WHO statement said, the risk of international spread of the disease cannot be ruled out.