The Ebola outbreak has now killed more than 4,000 people, according to the World Health Organization.

The UN health agency said Friday that the number of people known to have died has risen to 4,033 out of 8,399 cases in seven countries by the end of Oct. 8.

The deaths were in Liberia, Sierra Leone, Guinea, Nigeria and the United States.

Earlier on Friday, the UN’s special envoy for the disease said the world’s response to the Ebola crisis needs to be 20 times greater than it was at the beginning of the month.

Without mobilization on a massive scale, "it will be impossible to get this disease quickly under control, and the world will have to live with the Ebola virus forever," David Nabarro warned the UN General Assembly on Friday.

He urged all countries to act swiftly and generously, saying that contributions "within days" is more important than larger contributions within weeks.

Thousands of health workers, community mobilizers and people to construct and staff treatment centres are needed, Nabarro said, in calling for a scale-up in response. The outbreak presents an extraordinary challenge for the world that needs ingenuity and collective action against a virus that is invisible to our eyes and poorly understood by most.

The United Nations Mission for Ebola Emergency Response (UNMEER) said the three countries most affected in the outbreak — Guinea, Sierra Leone and Liberia — recently showed high economic growth rates, but those may be halved in a year.

A woman grieves after Ebola burial team members arrived to take away the body of Mekie Nagbe, 28, for cremation on Friday in Monrovia, the capital of Liberia. A backlog for burial of corpses is a significant source of spread of the virus, officials say. (John Moore/Getty)

Ebola is not just a health crisis, but a multidimensional one that kills, harms health systems, education, food security and livelihoods, Anthony Banbury, special representative and head of UNMEER, told the assembly.

"The world must now act to help the people and governments of Liberia, Sierra Leone and Guinea," Banbury said. "A failure to act now while we have the chance could lead to unpredictable but very dire consequences for the people of the countries and well beyond. As long as there is one case of Ebola in any one of these countries, no country is safe from the dangers posed by this deadly virus."

Liberia’s health minister said while the health and economic systems are currently devastated, the goal of relief efforts should be to leave behind a stronger health system.

A health official from Sierra Leone said most the affected patients are in the age group of 15 to 50. Challenges include:

Weak surveillance and contact tracing.

A lack of ambulance and other logistics to move cases from the community to treatment centres.

A shortage of hospital beds.

Too few customized laboratories to test specimens, which means holding centres have become a breeding ground for spread of the Ebola virus.

A backlog for burial of corpses, which are a significant source of spread of the virus.

Pockets of denial remain among people.

UN deputy secretary general Jan Eliasson said the agency’s appeal for $1 billion US to respond to the outbreak is only one-quarter funded.