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.

Liberia's President Ellen Johnson Sirleaf apologized on Saturday for the high death toll among the country's healthcare workers who have fought an Ebola outbreak, which has killed nearly 1,000 people in three countries.

Johnson Sirleaf pledged up to $18 million for the Ebola fight, part of which will be given to health workers to help with insurance and death benefits, to fund more ambulances and to increase the number of treatment centres.

"If we haven't done enough so far, I have come to apologize to you," she told hundreds of health workers who gathered at Monrovia's City Hall for a meeting with her government.

The West African Ebola outbreak, centred on Guinea, Sierra Leone and Liberia, is the worst in history. The World Health Organization (WHO) said on Friday it is an international health emergency that will likely continue spreading for months.

The disease has put a severe strain on the health systems of affected states and governments have responded with a range of measures, including the declaration of national emergencies in Sierra Leone, Liberia and Nigeria, which confirmed seven cases of Ebola in Lagos.

Ebola has reaped a high toll on health workers who have acted as first responders. Liberia alone has lost at least three doctors to the virus and 32 health workers.

Sierra Leone's Health Ministry said a senior physician had contracted the disease at the Connaught referral hospital in the capital, Freetown.

Dr. Modupeh Cole contracted the disease "after treating a patient ... who was later proved to have the virus and died," said ministry spokesman Sidi Yahya Tunis.

Cole was taken to an Ebola treatment centre in eastern Kailahun district, run by medical charity Medecins Sans Frontieres, Tunis said.

He is the latest Sierra Leonean medical practitioner to contract the virus. The country's leading Ebola doctor, Shek Umar Khan, died of the disease last month and several nurses have died.

GUINEA BORDERS TO STAY OPEN

Guinea said earlier on Saturday at a news conference attended by four government ministers that it had closed its borders with Sierra Leone and Liberia to halt the spread of Ebola.

Authorities said the decision was taken primarily to prevent infected people crossing into Guinea, where at least 367 people have died of Ebola since March and 18 others are being treated in isolation.

However, state television later said the borders remained open, in an about-face that appeared to highlight the difficulties governments face in coordinating policy in the face of the fast-moving outbreak.

"Guinea has not closed its borders with Sierra Leone or with Liberia. It's rather that we have taken health measures at the border posts," the television channel said.

A government source said the minister who made the original announcement had not been in possession of accurate information.

Ebola is one of the deadliest diseases known to humanity. It has no proven cure and there is no vaccine to prevent infection. The most effective treatment involves alleviating symptoms that include fever, vomiting and diarrhoea.

The rigorous use of quarantine is needed to prevent its spread, as well as high standards of hygiene for anyone who might come into contact with the disease.

These measures have proved hard to enforce given that Ebola has spread in rural parts of some of the world's poorest countries. The task is made harder because of mistrust of health workers in areas with inadequate public health services.