Ebola outbreak risk to US 'very low'
US National Security Adviser Susan Rice has said the risk of Ebola transmission in the US is "very low", as a second ailing American arrives for treatment.
One US aid worker is being cared for in Atlanta while his colleague, also infected, is on her way from Liberia to the same specialist hospital.
Another man, in New York, is being tested after travelling to the region.
Since February, 887 people have died in four West African countries. The World Bank has given $200m (£120m) to help.
Untested in humans
The Centers for Disease Control (CDC) in the US has received at least 100 calls from people questioning why the sick aid workers should be allowed into the country.
But Ms Rice told NBC the risk to Americans was deemed "very low" by the CDC.
"We have in this country the protocols to isolate and manage any patient who may present with those symptoms of the disease."
Nancy Writebol, 59, an aid worker infected with the virus in Liberia, is expected to arrive in Atlanta for treatment on Tuesday morning.
She is said to be progressing after receiving two doses of a experimental treatment not previously tested on humans.
Her husband, David Writebol, has told the president of the aid agency she was working for that she is weak but showing signs of improvement.
A week ago the family was considering funeral arrangements, but now they were relieved and cautiously optimistic about her condition, he said.
Her colleague, Dr Kent Brantly, 33, who also received the experimental treatment, arrived at Emory University Hospital on Saturday and was able to walk into hospital, where he is being held in isolation.
Known as ZMapp, the treatment boosts the immune system's ability to fight off Ebola through antibodies made by lab animals exposed to elements of the virus.
Analysis - Helen Briggs, health editor, BBC News website
A couple of experimental treatments have been used on a compassionate basis.
The approach is based on the idea that antibodies against the Ebola virus might help the body fight off the infection.
An antibody is a protein produced by the immune system in response to harmful invaders, such as viruses.
It had shown promise in monkeys, CNN reports, but had never before been tested on humans.
It is unclear if the Americans' improvements are related to the drug. Dr Brantly was reportedly also given a blood transfusion from a boy who recovered from Ebola while in his care.
In a statement, the company said very little of the drug is available and they were "co-operating with appropriate government agencies to increase production as quickly as possible".
The US Food and Drug Administration (FDA) must grant permission to use such treatments on humans in the US, but the agency does not have authority over its use outside the country.
An FDA spokeswoman could not confirm or deny FDA granting access to any experimental therapy for the aid workers inside the US, the Associated Press reports.
The two aid workers were serving with aid groups helping fight the Ebola outbreak in Liberia.
There is no cure or vaccine for Ebola, but patients have a better chance of survival if they receive early treatment.
The current outbreak is killing between 50% and 60% of people infected. Initial flu-like symptoms can lead to external haemorrhaging from areas like eyes and gums, and internal bleeding which can lead to organ failure.