By Skye Wheeler

MALAKAL, Sudan, Nov 6 (Reuters) - A deadly tropical disease has killed two people and sickened 375 in southern Sudan, aid agency Medecins Sans Frontiers (MSF) said on Friday, warning that many more were likely to be infected.

The parasitic kala azar disease is almost always fatal if untreated and can kill those with weak immune systems within weeks, MSF said in a statement, adding that 95 percent of patients survive with timely treatment.

"We suspect that the number of kala azar patients reaching clinics in some areas is just the tip of the iceberg," David Kidinda, MSF's South Sudan Medical Coordinator, said. At least two people have died, the statement added.

The disease, also known as leishmaniasis, is caused by a parasite transmitted through sand fly bites. It multiplies in the body, attacking the immune system, and symptoms include an enlarged spleen, fever, weakness and wasting, MSF said.

The disease is endemic to parts of southern Sudan. MSF said it treated 100 cases last year and that outbreaks occur every five to 10 years.

Poor roads and few hospitals, clinics or other medical services in the south, emerging from more than two decades of civil war, mean that outbreaks of disease are common. The bitter north-south war claimed 2 million lives before ending with a 2005 peace deal.

"In southern Sudan, where almost three quarters of the population have no access to even the most basic healthcare, it's a race against time to reach patients," Kidinda added.

(Editing by Michael Roddy)