An Ebola outbreak in Guinea that has killed 78 people has crossed borders into Liberia and Sierra Leona, health officials said.

A total of four people in Liberia and Sierra Leone are thought to have contracted the Ebolavirus while traveling to Guinea, according to the World Health Organization. At least three of them have died.

Senegal has closed its border crossings with Guinea until further notice, The Associated Press reported.

Two health care workers are among the 112 suspected cases in the growing outbreak, "indicating the need to further strengthen health facility-based infection prevention and control," WHO said in a statement.

The outbreak has sickened at least 112 people, according to WHO. (Image credit: Youssouf Bah/AP Photo)

At least four health care workers have been killed while working to contain the outbreak.

"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."

Inside the Ebola Ward: On the Front Lines in Uganda

Ebola is a deadly kind of hemorrhagic fever caused by a group of viruses. The virus in the Guinea outbreak has a fatality rate of 62.5 percent, according to WHO.

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.

Health officials are training community members in the West African countries to protect themselves while caring for loved ones and burying the dead, WHO said.

The Associated Press contributed to this story.