(CNN) Ebola virus disease has taken the lives of 27 people in the Democratic Republic of Congo, the World Health Organization said Wednesday. Based on information gathered this week, WHO also reported a total of 58 cases of the rare and deadly illness in the Congo outbreak.

Ebola virus disease, which causes fever, severe headache and in some cases hemorrhaging, most commonly affects people and nonhuman primates, such as monkeys, gorillas and chimpanzees. There are five subtypes of Ebola virus, four of which can be transmitted from wild animals to people, who can then spread the disease person-to-person.

The current outbreak is caused by the Zaire ebolavirus, which has the highest mortality rate, ranging from 60% to 90%, according to WHO.

The total case count includes 28 confirmed, 21 probable and nine suspected patients from three health zones in the country's northwestern province of Equateur: Bikoro, which reported the first and most cases, Iboko and Wangata.

The newest cases confirmed by WHO occurred in Wangata, a health zone in Mbandaka, a city on the Congo River and boasting a population of about a million.

The spread of the virus into this particular city, which is connected to Kinshasa and bordering countries via tributaries of the Congo River, has raised fears it could spread more quickly and become harder to control, according to Peter Salama , the deputy director general of the WHO's Health Emergencies Program.

Salama spoke during a briefing on the Ebola outbreak response for delegates assembled at the 71st Session of the World Health Assembly.

'Not the only challenge'

Ebola virus spreads through direct contact with either bodily fluids or objects contaminated by someone ill with the disease, according to the US Centers for Disease Control and Prevention. In some cases, the virus is spread from contact with someone who has died from the disease. The virus enters the body through broken skin or mucous membranes in the eyes, nose or mouth. People can get it through sexual contact, as well.

Three health care workers were among the 58 cases reported.

"It is critical to remember that the Ebola outbreak in northwest DRC is not the only challenge this country is facing," Salama said. "There is massive population displacement, food insecurity and malnutrition, particularly in the east of the country, and multiple simultaneous outbreaks of cholera, of measles and recently of vaccine-derived poliovirus," a rare mutated strain.

Pharmaceutical giant Merck has provided more than 8,000 doses of its experimental Ebola vaccine and will make an additional 8,000 doses available to WHO in the coming days.

As of Monday, the starting day of the WHO vaccination program, health care workers had identified over 600 people who have come into contact with those who have fallen ill, Salama said. WHO is working with the country's Ministry of Health, Médecins Sans Frontières (Doctors Without Borders) and other partners to inoculate only those people at high risk of infection: primary and secondary contacts.

"The detective work of epidemiology will make or break the response to this outbreak," Salama said.

'Enthusiastic and determined contributions'

Dr. Tedros Adhanom Ghebreyesus , director-general of WHO, noted the difficulty of maintaining the vaccine's proper temperature in country.

"The commitment and sacrifice of those on the front line is the most important element of this outbreak," he said. In particular, he emphasized the proximity of neighboring countries as a major challenge of the current outbreak.

Dr. Matshidiso Moeti, the WHO regional director for Africa, said the nine countries bordering Congo have initiated preparedness activities, which includes training health care workers and supplying clinics. Central African Republic, which is nearest to the affected Congo province, is of the greatest concern and is a priority for WHO.

"I have seen enthusiastic and determined contributions to the preparedness of these countries," Moeti said.

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Salama noted that, in eight Ebola outbreaks in Congo, two patterns have emerged: The virus affected either isolated rural areas only or towns only.

"This outbreak has features of both," Salama said, as disease has spread from villages and begun to encroach on larger towns and cities.

"We're really just at the beginning," he said. "The next few weeks will really tell if this outbreak is going to expand to urban areas or if we are going to be able to keep it under control."