TIME Health Reporter Alexandra Sifferlin explains why there's no cure for Ebola, how infected patients are treated, and more common questions about the disease.

The director of the U.S. Centers for Disease Control and Prevention said on Thursday he has activated the agency's emergency operation center at the highest response level to help respond to the worst Ebola outbreak in history, which has killed 932 people.

In testimony at a special congressional hearing on Ebola, CDC Director Dr. Thomas Frieden said the centers have more than 200 staff members in Atlanta working on the outbreak, and will soon have more than 50 disease experts in West Africa to try to contain the outbreak.

Although Frieden said it is possible that people who have traveled to West Africa might bring the virus back home with them, and even spread it to some healthcare workers and family members, he said, he is "confident there will not be a large Ebola outbreak in the United States."

The scale of the outbreak is unprecedented, both in terms of its organic spread and travel of those being treated for Ebola. Confirmed cases have been reported in four African nations - Guinea, Sierra Leone, Liberia and Nigeria - while aid workers have been evacuated to the United States and Spain for treatment. On Wednesday a suspected case of Ebola killed a Saudi man in Jeddah who recently visited Sierra Leone on business, meaning human cases of Ebola - once entirely confined to Africa - now likely have traveled to four continents.

A hospital in Benin is treating a Nigerian man suspected of having contracted Ebola and authorities have sent a sample of his blood to Senegal for testing, Health Minister Dorothée Gazard said on state television on Thursday.

The Benin case is unconfirmed but Gazard's announcement triggered widespread fears in the capital Cotonou. Many people said they would stock up on food and stop eating at popular roadside food stalls to avoid possible infection, witnesses said.

LIBERIA DECLARES EMERGENCY

Police and soldiers in Sierra Leone blockaded rural areas hit by Ebola on Thursday, a senior officer said, after neighboring Liberia declared a state of emergency to tackle the outbreak.

Liberian President Ellen Johnson-Sirleaf announced emergency measures late on Wednesday that will, for 90 days, allow her government to curtail civil rights by imposing quarantines on badly affected communities to contain the epidemic.

Though the vast majority of cases are in the remote border area of Guinea, Sierra Leone and Liberia, concern over Ebola's spread grew last month when a U.S. citizen died in Nigeria of the virus after arriving from the region. A nurse who treated him has now also died in Lagos, and at least five other people have been isolated with symptoms.

Some major airlines, such as British Airways and Emirates, have halted flights to affected countries, while many expatriates are leaving, officials have said.

In eastern Sierra Leone - the worst-hit area of the country - the head of police said security forces deployed last night "to establish a complete blockade" of Kenema and Kailahun districts, setting up 16 checkpoints on major roads.

"No vehicles or persons are allowed into or out of the districts," Alfred Karrow-Kamara told Reuters, saying the measures would last for an initial 50-day period.

He said traders who had registered with security agencies would be able to bring in food and medicines. Security forces would mount foot patrols to ensure civilians did not slip past their roadblocks through the bush. 

Among some of the world's poorest states, Sierra Leone, Liberia and Guinea had shown signs of progress, leaving behind bloody civil wars fueled by post-colonial poverty. How much Ebola permanently hampers the sputtering economies remains to be seen. The outbreak is hampering transport of goods, tourism, and pulling workers from their mines and farm fields, either to care for the sick or to avoid falling ill themselves. 

Liberian and Sierra Leonean officials appealed to major investors not to abandon their countries because of Ebola.

"My message is: 'Don't leave the country. Stay with us - let's fight this together'," Liberia's Finance Minister Amara Konneh said.

However, the United States ordered families of its diplomats in Liberia to leave and warned against non-essential travel to the West African country because of the growing Ebola outbreak.

The State Department said U.S. staff would remain on active duty at the embassy and additional staff were being sent to help the government tackle the outbreak of the deadly virus.

"The latest wave of the outbreak has overwhelmed Liberia's health system and most health facilities lack sufficient staff or resources to address the continuing transmission" of the disease, it said.

Extra U.S. personnel going to Monrovia include 12 disease prevention specialists from the Centers for Disease Control and Prevention and a 13-member disaster assistance response team from USAID to help the government fight the outbreak, State Department spokeswoman Marie Harf said.

Liberian President Ellen Johnson-Sirleaf announced a state of emergency on Wednesday effective for 90 days that allows the government to curtail civil rights and deploy troops and police to impose quarantines on badly affected communities.

The epidemic has also hit Sierra LeoneGuinea and Nigeria and has claimed more than 900 lives, according to the World Health Organization.