The latest Ebola virus outbreak has killed more than 100 people in West Africa and up to 175 more people may have been infected, as the death toll has seen a sharp increase in the last week.

“This is one of the most challenging Ebola outbreaks that we have ever faced,” said Dr. Keiji Fukuda, Assistant Director-General of the Health Security and Environment cluster at the WHO, at a press conference in Geneva on Tuesday. Fukuda said that the two reasons for the quick spread of Ebola are the wide geographic dispersion of cases, and because it’s a “quite lethal infection."

The WHO said that Guinea and Liberia are the only two countries with confirmed cases of the virus, but that it's nevertheless an “international outbreak.” In Guinea, there have been 101 deaths, and in Liberia there have been 7 deaths, said Dr. Stéphane Hugonnet, a medical officer with WHO. While there were new fears last week that Ebola had spread to Mali, the two cases tested in the country were negative. There are also no Ebola cases in Ghana, and no cases in Sierra Leone, according to the WHO.

“These kind of outbreaks are often surrounded by a great deal of fear and anxiety, creating rumours and making communications challenging and important,” Fukuda said.

People infected with Ebola have virtually zero chance of survival, and Hugonnet said that nine out of ten of those infected are dying. The virus is transmitted through body fluids or close contact with an infected person, but authorities say some family members have contracted the disease after handling a corpse during burial. WHO have deployed 50 experts and support staff to the areas of infection, and 17 more will be deployed shortly.