West Africa is struggling with the worst Ebola outbreak on record, and two U.S. nurses have contracted the disease treating a Liberian man who died in Texas.

A nurse in Spain was infected while caring for two priests, who both died of Ebola.

Below are some facts regarding the outbreak:

- Ebola has killed 4,493 people, or about 50 percent of the known 8,997 cases, predominantly in Liberia, Sierra Leone and Guinea. Ebola, which emerged in a remote forest region of Guinea in March, has also turned up in Nigeria and Senegal, but officials say the disease has been contained in those two countries.

- There is no vaccine or cure for Ebola, a hemorrhagic fever. In past outbreaks, fatality rates have reached up to 90 percent. Ebola causes fever, flu-like pains, bleeding, vomiting and diarrhea.

- Pharmaceutical companies are working on experimental Ebola vaccines and antiviral drugs, but a significant number of doses will not be available until at least the first quarter of 2015.

- Ebola is not airborne. It is transmitted through blood, vomit, diarrhea and other bodily fluids. Healthcare workers in West Africa have been among the hardest hit by the outbreak.

- Ebola symptoms generally appear between two and 21 days after infection, meaning there is a significant window during which an infected person can escape detection, allowing them to travel. However, they are not considered contagious until they start showing symptoms.

- Recovery from Ebola depends on the patient’s immune response. People who recover from Ebola infection develop antibodies that last for at least 10 years.

- The U.S. Centers for Disease Control and Prevention (CDC) has estimated that the number of infections could rise to up to 1.4 million people by early next year without a massive global intervention to contain the virus.

- The United States, Britain, France, China, Cuba and international organizations are pouring funds, supplies and personnel into the affected parts of West Africa.

- Ebola's suspected origin is forest bats. The virus was first identified in 1976 in what is now known as Democratic Republic of the Congo.