From 1976 (when it was first identified) through 2013, fewer than 1,000 people per year have been infected. [ 1 ] [ 4 ]The largest outbreak to date is the ongoing 2014 West Africa Ebola outbreak, which is affecting Guinea, Sierra Leone, Liberia, and Nigeria. [ 5 ] [ 6 ]As of August 2014, more than 1,750 suspected cases have been reported. [ 7 ]Efforts are going on to develop a vaccine; however, none yet exists. [ 1 ]
Signs and symptoms
Symptoms of Ebola. [ 8 ]
Signs and symptoms of Ebola usually begin suddenly with am influenza-like stage characterized by fatigue, fever, headaches, and joint, muscle, and abdominal pain. [ 9 ] [ 10 ]Vomiting, diarrhea and loss of appetiteare also common. [ 10 ]Less common symptoms include: sore throat, chest pain, hiccups, shortness of breath, and trouble swallowing. [ 10 ]The average time between contracting the infection and the start of symptoms is 8 to 10 days, but it can vary between 2 and 21 days. [ 10 ]Skin manifestations may include a maculopapular rash(in about 50% of cases). [ 11 ]Early symptoms of EVD may be similar to those of malaria, dengue fever, or other tropical fevers, before the disease progresses to the bleeding phase. [ 9 ]
In 40–50% of cases, bleeding from puncture sites and mucous membranes(e.g. gastrointestinal tract, nose, vagina, and gums) has been reported. [ 12 ]In the bleeding phase, which typically starts 5 to 7 days after first symptoms [ 13 ]internal and subcutaneous bleeding may present itself through reddening of the eyesand bloody vomit. [ 9 ]Bleeding into the skin may create petechiae, purpura, ecchymoses, and hematomas(especially around needle injection sites). Types of bleeding known to occur with Ebola virus disease include vomiting blood, coughing it upor blood in the stool. Heavy bleeding is rare and is usually confined to the gastrointestinal tract. [ 11 ] [ 14 ]In general, the development of bleeding symptoms often indicates a worse prognosis and this blood loss can result in death. [ 9 ]All people infected show some symptoms of circulatory systeminvolvement, including impaired blood clotting. [ 11 ]If the infected person does not recover, death due to multiple organ dysfunction syndromeoccurs within 7 to 16 days (usually between days 8 and 9) after first symptoms. [ 13 ]
Causes
Main articles: Ebolavirus (taxonomic group)and Ebola virus (specific virus)
Life cycles of the Ebolavirus
EVD is caused by four of five viruses classified in the genus Ebolavirus, family Filoviridae, order Mononegavirales. The four disease-causing viruses are Bundibugyo virus(BDBV), Sudan virus(SUDV), Taï Forest virus(TAFV), and one called simply, Ebola virus(EBOV, formerly Zaire Ebola virus)). Ebola virus is the sole member of theZaire ebolavirusspecies, and the most dangerous of the known Ebola disease-causing viruses, as well as being responsible for the largest number of outbreaks. [ 15 ]The fifth virus, Reston virus(RESTV), is not thought to be disease-causing in humans. The five Ebola viruses are closely related to the Marburg viruses.
Transmission
It is not entirely clear how Ebola is spread. [ 16 ]EVD is believed to occur after an ebola virus is transmitted to an initial human by contact with an infected animal's body fluids. Human-to-human transmission can occur via direct contact with blood or bodily fluids from an infected person (including embalmingof an infected dead person) or by contact with contaminated medical equipment, particularly needles and syringes. [ 17 ]Semen is infectious in survivors for up to 50 days. Transmission through oral exposure and through conjunctivaexposure is likely [ 18 ]and has been confirmed in non-human primates. [ 19 ]The potential for widespread EVD infections is considered low as the disease is only spread by direct contact with the secretions from someone who is showing signs of infection. [ 17 ]The quick onset of symptoms makes it easier to identify sick individuals and limits a person's ability to spread the disease by traveling. Because dead bodies are still infectious, some doctors disposed of them in a safe manner, despite local traditional burial rituals. [ 20 ]
Medical workers who do not wear appropriate protective clothing may also contract the disease. [ 21 ]In the past, hospital-acquiredtransmission has occurred in African hospitals due to the reuse of needles and lack of universal precautions. [ 22 ] [ 23 ]
Airborne transmission has not been documented during previous EVD outbreaks. [ 2 ]They are, however, infectious as breathable 0.8– to 1.2-μm laboratory-generated droplets; [ 24 ]because of this potential route of infection, these viruses have been classified as Category A biological weapons. [ 25 ]Recently, the virus has been shown to travel without contact from pigs to nonhuman primates, although the same study failed to achieve transmission in that manner between primates

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