The outbreak of Ebola in Guinea, Liberia and Sierra Leone is the worst ever, this year infecting more than 6,500 people, killing almost 3,100 of those patients. Below are frequently asked questions about the disease.

Q: How does an outbreak of Ebola start and spread?

A: The origin of the virus is unknown, but fruit bats are considered the most likely host, according to the World Health Organization. Ebola jumps to humans from infected animals including chimpanzees, gorillas and bats that live in the rain forest, through contact with blood or other bodily fluids such as urine and saliva. Infection can also occur if broken skin or mucous membranes of a healthy person come into contact with items that have become contaminated with an Ebola patient's infectious fluids such as soiled clothing, bed linen, or used needles.

Q: What are the chances of the outbreak spreading beyond Africa?

A: One American has been diagnosed with the disease after traveling in Liberia, and U.S. public health officials are tracking anyone he had contact with. Three other American aid workers infected in Liberia were evacuated to U.S. hospitals, where they were treated, recovered and released. The Centers for Disease Control and Prevention has said an Ebola outbreak in the United States similar to the one in west Africa is highly unlikely because the U.S. has better infection controls in hospitals and in burial procedures.

Q: Is there any treatment or cure for Ebola?

A: There is currently no specific treatment to cure the disease. Severely ill patients require intensive supportive care. They are frequently dehydrated and need intravenous fluids or oral rehydration with solutions that contain electrolytes.

Q: Can coughing and sneezing spread the virus?

A: While an infected person who sneezes or coughs directly in another person's face could infect that person, Ebola primarily enters the body through tiny cuts or abrasions, or through mucus membranes of the eyes, nose, ears and mouth.

Q: What are the symptoms of the virus?

A: Sudden onset of fever, intense weakness, muscle pain, headache and sore throat are typical signs and symptoms. This is followed by vomiting, diarrhea, rash, impaired kidney and liver function, and in some cases, internal and external bleeding. Infections can only be confirmed through laboratory testing.

Q: How is this outbreak different from past outbreaks?

A: The current outbreak in Guinea, Sierra Leone and Liberia is the first time Ebola has appeared in West Africa. Until now, the Democratic Republic of Congo and Uganda have seen the worst outbreaks, which have tended to be in isolated areas in single countries. A lack of border control has allowed infected people who didn't seek medical attention because of fear, suspicion or stigma to travel freely between the three countries.

Q: How long does it take for symptoms to appear?

A: In the past, the incubation period has ranged from 2 days to 21 days. In the current outbreak, it has been 4 days to 6 days, according to the WHO.

Q: Where do we stand with developing drugs for Ebola?

A: An experimental antibody cocktail being developed by the U.S. government, the Public Health Agency of Canada and two drug companies, Mapp Biopharmaceutical Inc. of San Diego and Toronto- based Defyrus Inc. has shown promise in animal tests. Called ZMapp, the drug, produced using tobacco plants, hasn't been tested in humans, but was approved by the Food and Drug Administration for emergency use for two of the infected American health workers who recovered. Safety trials in healthy humans may begin in the first half of next year, according to Defyrus. Tekmira Pharmaceuticals Corp. also is testing its Ebola therapy, which was given to the third U.S. aid worker who recovered.

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