As the biggest-ever outbreak of Ebola continues to ravage West Africa, here are a few key numbers to get a handle on the epidemic:

13,042 and 4,818:

According to an update this week from the World Health Organization, there have been 13,042 Ebola cases and 4,818 deaths since the first child died of the virus in December - but those figures include all probable, suspected and confirmed cases and are subject to change as more information becomes available. The numbers have actually dropped slightly in the past week because numbers fluctuate as more data becomes available, and as probable and suspected cases are either discarded or confirmed.

Experts warn the number of cases and deaths are likely far below the actual numbers, as people may be reluctant to seek care and officials are too overwhelmed with control efforts to record every single case. The vast majority of patients are in Guinea, Sierra Leone and Liberia.

Cases and deaths are typically only recorded days after people become symptomatic or die, which complicates a real-time understanding of Ebola.

"We are definitely getting a delayed picture of the outbreak," said Sebastian Funk, a lecturer in infectious diseases at the London School of Hygiene and Tropical Medicine. "It's difficult to tell if we are reaching a turning point or if there will be a doomsday scenario. I could see it going either way at the moment."

4,707:

According to WHO, 4,707 beds are needed across West Africa in Ebola treatment clinics; at the moment, just 22 percent of the necessary number are operational. The agency estimates a further 2,685 beds are needed for basic Ebola clinics where minimal treatment is provided and people are mostly isolated while waiting for test results. At the moment, just 4 percent of beds in these community clinics are available.

Given the uncertainty around case numbers, Funk says things like bed occupancy are a good sign of how the outbreak is evolving. "We really need to see numbers go down for several weeks to be confident it's a real trend," he said.

546:

WHO reports that 546 health workers have been infected with Ebola, of whom 310 have died. Since Ebola is spread via contact with the bodily fluids of a patient, health workers are at high risk of catching the disease. The U.S. Centers for Disease Control and Prevention recommends that no skin be showing in a health worker treating Ebola patients.

Eight:

There are now eight countries that have reported Ebola detected on their soil: Guinea, Liberia, Sierra Leone, Mali, Nigeria, Senegal, Spain and the U.S. Other countries like Britain, France and Germany have taken in people sickened by the deadly virus in West Africa but they have not reported any spread of the virus there.

Four:

There have been four cases of Ebola in the U.S. and a single death - Thomas Eric Duncan, the Liberian man who was the first person diagnosed with Ebola in the country, who later died at a Dallas hospital. Three other cases of Ebola have been reported in the U.S., all in health workers who had direct exposure taking care of Ebola patients.

Two:

It took two months for Britain to build and open its first Ebola treatment center in Sierra Leone this week, an 80-bed facility with a dozen other beds reserved for infected health workers. The Kerry Town clinic, near the capital, Freetown, is the first of six centers to be built by the U.K. in Sierra Leone. It includes a triage center, ambulance pads, laboratory, pharmacies and decontamination units. In about the same time that it took to build the Ebola clinic, Sierra Leone has reported more than 3,500 cases of Ebola.

One:

There is one remaining American hospitalized with Ebola, Dr. Craig Spencer in New York, a physician who worked for Doctors Without Borders in Guinea. He continues to show improvement, remains in isolation and is in stable condition.