Ebola crisis: How did the virus get into the US?

A Liberian man is the first person to be diagnosed with the deadly Ebola virus on US soil. So how did he arrive undetected and what are the risks to Americans?

The outbreak gripping West Africa is the world's worst, killing 3,338 people so far.

There have been 7,178 confirmed cases, with Sierra Leone, Liberia and Guinea suffering the most.

This case in the US is thought to be the first outside West Africa involving this strain of Ebola.

Who is the new patient in the US?

Only one person has been diagnosed with the virus while in the US.

Thomas Eric Duncan, a Liberian national, tested positive in Dallas, Texas, on 30 September.

He arrived in the US to visit relatives on 20 September and is now in a serious condition in an isolation unit at a hospital in Dallas.

Is he the first person in the US to have Ebola?

No, a small number of American aid workers who contracted the virus while abroad have recovered after flying back to the US for treatment. But Mr Duncan is the first diagnosed within the US.

Three of the aid workers were colleagues at the same hospital in Liberia. Kent Brantly and Nancy Writebol were flown back to Atlanta for treatment, while Rick Sacra, a family doctor from Massachusetts, recovered in Nebraska.

Ms Writebol said resources at the hospital where they worked were insufficient to protect workers.

A month after returning to the US, Ms Writebol was well enough to speak to reporters

Do we know how Mr Duncan was infected?

It is believed he came into contact with an Ebola infectious woman in Liberia on 15 September, according to a report by the New York Times.

Mr Duncan is said to have helped take her to hospital, but she was later turned away due to lack of space in the Ebola treatment ward. He helped to carry her home, where she died hours later.

Williams' brother, Sonny Boy, also later displayed symptoms of Ebola and died en route to a local hospital.

How did an infected person come into the US undetected?

Mr Duncan was screened for Ebola symptoms at Roberts International Airport, located near the Liberian capital of Monrovia.

He displayed no signs of the virus and was allowed to board an SN Brussels Airlines flight to Brussels, Belgium.

From there, he flew to Washington Dulles and then on to Dallas-Fort Worth, arriving on 20 September.

Binyah Kesselly, chairman of the board of the Liberia Airport Authority, said they had screened 10,000 passengers since July, but it would be "nearly impossible" to identify a person as infected with the Ebola virus if they were not showing symptoms.

Are the passengers on his flight in danger?

The Centers for Disease Control and Prevention (CDC) do not plan to monitor passengers on Mr Duncan's flights.

According to CDC Director Thomas Frieden, Mr Duncan was not considered infectious at that time and posed "zero risk of transmission" to those on the aircraft.

How dangerous is Ebola anyway?

• Symptoms include high fever, bleeding and central nervous system damage
• Spread by body fluids, such as blood and saliva
• Fatality rate can reach 90% - but current outbreak has mortality rate of about 70%
• Incubation period is two to 21 days
• There is no proven vaccine or cure
• Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
• Fruit bats, a delicacy for some West Africans, are considered to be virus's natural host

Ebola virus: Busting the myths

Could it now spread in the US?

Health officials have expressed confidence they can contain the virus.

They say it's not comparable to conditions in Africa because basic sanitation levels are so different.

"The bottom line here is I have no doubt that we will control this importation, this case of Ebola, so it does not spread widely in this country," said Mr Frieden. "We will stop it here."

But they are closely monitoring all those who may have come into contact with Mr Duncan while he was infectious.

The hour after America knew

Should he have been diagnosed earlier?

Mr Duncan sought medical attention at Texas Health Presbyterian Hospital on 25 September, where hospital officials said he displayed a low grade fever and abdominal pain. Basic blood tests were performed, but he was not screened for the Ebola virus.

A nurse asked Mr Duncan if he had travelled from Africa, and he said he had, but that detail was not fully communicated to the medical staff, an oversight the hospital now says it "regrets".

Mr Duncan was given antibiotics and a pain reliever and sent home, where his condition worsened, says his sister.

On 28 September, a friend of Mr Duncan's contacted the CDC for advice, and was instructed to call the Texas Department of Health, who sent an ambulance.

Are people he has been in contact with at risk?

One of the children attends Sam Tasby Middle School

There was a four-day period between Mr Duncan developing symptoms and being put in isolation in hospital.

So officials are identifying and monitoring all the people he came into contact with. If they show no symptoms after 21 days, they are considered to be uninfected.

Up to 20 people, including five children, are being monitored after coming into contact with him, mostly at the house where he stayed.

According to a statement from the local school district in Dallas, Mr Duncan came into contact with five children from four different schools.

Mr Frieden said it is possible a family member who came in direct contact with the patient may develop Ebola in the coming weeks.