They were infected while working in Liberia, one of four West African nations dealing with the world's largest Ebola outbreak. On Monday, the World Health Organization said the death toll had increased from 729 to 887 deaths in Guinea, Sierra Leone, Liberia and Nigeria, and that more than 1,600 people have been infected.

In a worrisome development, the Nigerian Health Minister said a doctor who had helped treat Patrick Sawyer, the Liberian-American man who died July 25 days after arriving in Nigeria, has been confirmed to have the deadly disease. Tests are pending for three other people who also treated Sawyer and are showing symptoms.

There is no vaccine or specific treatment for Ebola, but several are under development.

The experimental treatment the U.S. aid workers are getting is called ZMapp and is made by Mapp Biopharmaceutical Inc. of San Diego. It is aimed at boosting the immune system's efforts to fight off Ebola and is made from antibodies produced by lab animals exposed to parts of the virus.

In a statement, the company said it was working with LeafBio of San Diego, Defyrus Inc. of Toronto, the U.S. government and the Public Health Agency of Canada on development of the drug, which was identified as a possible treatment in January.

The statement says very little of the drug is available and they are ``cooperating with appropriate government agencies to increase production as quickly as possible,'' but gives no details on who else might receive it or when.

The U.S. Food and Drug Administration must grant permission to use experimental treatments in the United States, but the FDA does not have authority over the use of such a drug in other countries. The aid workers were first treated in Liberia. An FDA spokeswoman said she could not confirm or deny FDA granting access to any experimental therapy for the aid workers while in the U.S.

Writebol, 59, has been in isolation at her home in Liberia since she was diagnosed last month. She's now walking with assistance and has regained her appetite, said Bruce Johnson, president of SIM USA, the group she works for in Africa. Writebol received two doses of the experimental treatment while in Liberia.

Johnson was hesitant to credit the treatment for her improvement.

``Ebola is a tricky virus and one day you can be up and the next day down. One day is not indicative of the outcome,'' he said. But ``we're grateful this medicine was available.''

Brantly, 33, who works for the international relief group Samaritan's Purse, also was said to be improving. Besides the experimental dose he got in Liberia, he also received a unit of blood from a 14-year-old boy, an Ebola survivor, who had been under his care. That seems to be aimed at giving Brantly antibodies the boy may have made to the virus.

Samaritan's Purse initiated the events that led to the two workers getting ZMapp, according to a statement Monday by the National Institute of Allergy and Infectious Diseases, part of the U.S. National Institutes of Health. The group contacted U.S. Centers for Disease Control and Prevention officials in Liberia to discuss various experimental treatments and was referred to an NIH scientist in Liberia familiar with those treatments.

The scientist referred them to the companies but was not officially representing the NIH and had no ``official role in procuring, transporting, approving, or administering the experimental products,'' the statement says.

In the meantime, dozens of African heads of state were in Washington for the U.S.-Africa Leaders Summit, a three-day gathering hosted by President Barack Obama. U.S. health officials on Monday spoke with Guinean President Alpha Conde and senior officials from Liberia and Sierra Leone about the Ebola outbreak.

The U.S. military has no biological weapons program but continues to do research related to infectious diseases as a means of staying current on potential threats to the health of troops.

The hospital in Atlanta that will treat the aid workers has one of the nation's most sophisticated infectious disease units. Patients are sealed off from anyone not in protective gear. Ebola is only spread through direct contact with an infected person's blood or other bodily fluids, not through the air. (AP)