(Photo Credit: Issac Kasamani/ AFP/Getty Images)

Four health care workers are among the 66 people who have died in the West African Ebola outbreak, according to the World Health Organization.

The ongoing outbreak has sickened 103 people in Guinea in all, and this Ebola strain has a 64 percent fatality rate, WHO officials said. The number of people reported sickened by Ebola in Guinea has more than doubled in the past five days.

Sierra Leone and Liberia have reported suspected cases and deaths among people who traveled to Guinea, according to WHO, which first reported the outbreak May 22.

"Intensive case investigations are underway to identify the source and route of these patients' infection, record their travel histories before arrival in Conakry [Guinea's capitol city] and determine their period of infectivity for the purposes of contact tracing," WHO said in a statement.

Canadian Man Tests Negative for Ebola

Ebola is a deadly kind of hemorrhagic fever caused by a group of viruses. The virus suspected in the Guinea outbreak is the Zaire ebolavirus, which was last seen in in the Democratic Republic of Congo in 2009. It's the deadliest ebolavirus, with a fatality rate approaching 90 percent.

The virus is transmitted through contact with blood or secretions from an infected person, either directly or through contaminated needles or medical equipment. The term "hemorrhagic fever" means it causes bleeding inside and outside the body. There's no cure.

Inside the Ebola Ward: On the Front Lines in Uganda

"There's nothing quite as frightening as stepping into an Ebola ward, knowing that one mistake, one slip of a mask or a glove, might lead to an untreatable deadly disease," said ABC News' chief health and medical editor Dr. Richard Besser, who traveled to Uganda in 2012 during an Ebola outbreak. "The medical teams and public health investigators working in Guinea right now are true heroes. They know that their work is critically important and is essential to saving lives."

ABC News' Katie Moisse contributed to this report.