The Ebola outbreak in West Africa is the world's deadliest to date. According to the UN, there have been approximately 1,100 cases in which 729 people have died as health officials in Guinea, Liberia and Sierra Leone struggle to control the virus. Liberia has closed most of its borders, banned public gatherings and announced quarantines of some communities in an effort to contain an outbreak of the Ebola virus. The current outbreak started in south-eastern Guinea in February and spread to Liberia in March and Sierra Leone in May. Last week Nigeria reported its first probable case, and was put on high alert after a Liberian man died in its most populous city, Lagos. Outbreaks are only considered to be over after 42 days (twice the maximum incubation period) without any new confirmed cases, meaning the current outbreak will continue until at least September. Today, as a hospital in Atlanta prepares for the return of an unnamed US aid worker infected with the deadly virus, the rest of the world is starting to wake up to the danger on its doorstep. We're not out of the woods quite yet. What is Ebola? Ebola virus disease (EVD) or Ebola haemorrhagic fever (EHF) is the human disease caused by the Ebola virus. The incubation period is typically between 2 days and 3 weeks. Symptoms initially include a fever, throat and muscle pains, and headaches. This then typically progresses to nausea, vomiting, and diarrhea, along with decreased functioning of the liver and kidneys. At this point, some people begin to have problems with haemorrhaging. How does it spread? The disease is usually acquired when a person comes into contact with the blood or bodily fluids of an infected animal such as a monkey or fruit bat. Why is this outbreak so worrying? -There is no specific treatment for the virus. As of 2014, no vaccine exists. Efforts to help persons who are infected include giving them either oral rehydration therapy or intravenous fluids. -The disease has a high mortality rate: often between 50% and 90% of those who are infected with the virus will die.