(CNN) -- Hundreds dead. Many more infected. Pervasive fear and denial are challenging authorities in west Africa who are trying to assess and address the Ebola crisis. 
 
 So far, it has killed more than 650 people, says Doctors Without Borders. 
 
 That's the highest death toll the World Health Organization has recorded in an Ebola outbreak. And it's getting worse. 
 
 The swelling numbers prompted heads of state to cancel travel plans on Thursday to direct their full attention toward fighting the outbreak of the virus that has crippled parts of Liberia, Sierra Leone and Guinea and stirred palpable concerns that it will spread around the region and the world. 
 
 Liberian President Ellen Johnson Sirleaf and Sierra Leone's President Ernest Koroma both canceled trips to the United States, and Koroma declared a state of emergency. 
 
 He announced a action plan that addressed many of the barriers international medical workers complain they face while fighting disease. 
 
 Some residents in affected villages have accused them of bringing the disease into the country and have barricaded their towns or otherwise blocked access to Ebola hemorrhagic fever victims. 
 
 A nurse with Doctors Without Borders, Monia Sayah, told CNN, "the most challenging" aspect of trying to help people is that "we go into communities where we are not necessarily welcome," because people don't want to believe they or their loved ones have Ebola -- in part because "they understand now that the survival rate is not very high." 
 
 Koroma said he will deploy police and military to accompany the aid workers. 
 
 They will search house to house for the infirm and enforce orders designed to curb the virus' spread. 
 
 What is the risk of catching Ebola on a plane? 
 
 Aid groups pull out 
 
 "The matter has reached a crisis point," Liberian Information Minister Lewis Brown told CNN's Richard Quest. "...The dire prognosis is that it will get worse before it gets better." 
 
 The dangers are so real that some humanitarian organizations are pulling out to protect their own. 
 
 Samaritan's Purse -- an international evangelical Christian humanitarian agency -- and the missionary group Serving in Mission have recalled all nonessential personnel from Liberia. 
 
 The Peace Corps announced Wednesday it is doing the same, removing its 340 volunteers from that country, as well as Sierra Leone and Guinea. 
 
 While there are no confirmed cases, a spokeswoman for the agency did say that two volunteers did come in contact with someone who ended up dying from the virus. 
 
 Those Americans haven't shown signs of Ebola but are being isolated just in case, with the spokeswoman saying they can't return home until they get medical clearance to do so. 
 
 American dies 
 
 One American, 40-year-old Patrick Sawyer, died in a Nigerian hospital earlier this month -- having come from Liberia, where he was a top Ministry of Finance official, and before he could go back home to Minnesota to celebrate his daughters' birthdays. 
 
 But the vast majority of those afflicted are Africans. They come from big cities and small villages, some of them falling ill without really knowing what hit them. 
 
 "This epidemic is without precedent," said Bart Janssens, director of operations for Doctors Without Borders, a group also known as Médecins Sans Frontières. "It's absolutely not under control, and the situation keeps worsening." 
 
 As of now, the outbreak has been confined to west Africa. But there are rising concerns that it could spread, especially since a person may not know they have Ebola or show symptoms for two to 21 days after being infected. 
 
 Sawyer, for example, collapsed getting off a plane in Lagos, Nigeria. He very well could have made it out of the region, perhaps to the United States, before showing symptoms of Ebola; it's only then that the virus spreads. 
 
 To further complicate matters, signs of Ebola include fever, headaches, weakness and vomiting -- symptoms that also define many other ailments, from malaria to the flu that Brown notes often pop up "at this time of year." 
 
 For all these reasons and more, Janssens says, "If the situation does not improve fairly quickly, there is a real risk for new countries to be affected." 
 
 Could outbreak spread to U.S.? 
 
 2 infected Americans 
 
 Ebola spreads through the transmission of bodily fluids. Those most at risk are loved ones of those infected, as well as health care workers tending to the ill. 
 
 Sawyer is believed to have been infected by his ailing sister, who he spent time with in Liberia -- even though neither likely knew she had Ebola -- according to Brown. 
 
 Then there are those like Dr. Sheik Humarr Khan, who fell ill early last week while overseeing Ebola treatment at a Sierra Leone hospital and died days later. 
 
 Two Americans affiliated with Samaritan's Purse who also were infected "have shown a slight improvement in the past 24 hours" though both are in serious condition, according to the Christian humanitarian agency. 
 
 One of them is Dr. Kent Brantly, a 33-year-old who last lived in Fort Worth, Texas. He has been the medical director for the Ebola Consolidated Case Management Center in Monrovia, Liberia, where he has been providing care for Ebola patients since October. After testing positive for the virus, Brantly went into treatment at ELWA Hospital. 
 
 The other is Charlotte, North Carolina, resident Nancy Writebol, a Serving in Mission member working with Samaritan's Purse to help fight the Ebola outbreak. 
 
 Ebola fears hit close to home 
 
 It is believed one of the local staff was infected with Ebola and came to work with the virus on July 21 and 22, Samaritan's Purse Vice President Ken Isaacs said. That staff member died Thursday. 
 
 "We think it was in the scrub-down area where the disease was passed to both Nancy and Kent," he said. 
 
 Brown, the Liberian information minister, noted Brantly and Writebol's fight in his CNN interview, as well as Liberia's need for more health care workers like them. 
 
 "We join the families in prayers that they can come through this and become ... shining examples that, if care is taken, one can come out of this." 
 
 Liberian president: 'Ebola kills' 
 
 On Monday, the CDC issued an alert warning travelers to avoid hospitals with Ebola patients and funerals for those patients in Liberia, Sierra Leone and Guinea due to the outbreak. 
 
 The United States is considering raising the alert to discourage "nonessential" travel to those three countries, a spokesman said. 
 
 Liberian President Ellen Johnson Sirleaf has told the Ministry of Health to consider quarantines in some areas and cremating bodies in an attempt to prevent further infection, according to Brown. 
 
 The president also urged residents to avoid public amusement and entertainment areas, and set aside Friday "for the disinfection and chlorination of all public facilities." 
 
 "My fellow Liberians, Ebola is real. Ebola is contagious. And Ebola kills," Sirleaf said. "All of us must all take extra measures announced by the Ministry of Health to keep ourselves safe. 
 
 "The government will do its part. But you must do yours." 
 
 READ: Ebola doctor in Sierra Leone dies 
 
 READ: What you need to know 
 
 CNN's Jonathan Helman, Elise Labott, Pamela Brown and Nana Karikari-apau contributed to this report. ||||| MONROVIA, Liberia (AP) — Liberia's president late Wednesday ordered the nation's schools to shut down and most civil servants to stay home as an Ebola outbreak that already has killed more than 130 people in the country deepened. 
 
 Meanwhile, the U.S. Peace Corps said it was evacuating its volunteers from Liberia as well as neighboring Guinea and Sierra Leone as the regional death toll topped 670 people. The Ebola outbreak is now the largest recorded in history. 
 
 President Ellen Johnson Sirleaf, who is skipping a summit of African leaders in Washington this week amid the crisis, also called for the closure of markets in an area near the borders with infected countries Guinea and Sierra Leone. 
 
 "My fellow Liberians, Ebola is real, Ebola is contagious and Ebola kills," she warned. "Denying that the disease exists is not doing your part, so keep yourselves and your loved ones safe." 
 
 Fear and panic over the mounting death toll has prompted some rural communities to accuse foreign aid workers of bringing the deadly virus. Others have kept people with Ebola symptoms at home instead of bringing them to quarantine centers. In anger, one man recently set fire to part of the health ministry building in the Liberian capital, Monrovia, after his teenage brother reportedly died of Ebola. 
 
 Sirleaf said that security forces would enforce the new precautions taken a week after an American man of Liberian descent boarded a plane in Monrovia and flew to Nigeria, where authorities said he died of Ebola. The fact that he was able to board a plane and traveled through a major airport transit hub in Togo has only heightened fears about Ebola's possible spread in the region. 
 
 The airline involved, ASKY, has suspended its flights to both the capitals of Liberia and Sierra Leone, and said passengers departing from Guinea would be carefully screened. 
 
 Ebola has no vaccine and no specific treatment, with a fatality rate of at least 60 percent. But experts say the risk of travelers contracting it is considered low because it requires direct contact with bodily fluids or secretions such as urine, blood, sweat or saliva. Ebola can't be spread like flu through casual contact or breathing in the same air. 
 
 Patients are contagious only once the disease has progressed to the point they show symptoms, according to the World Health Organization. The most vulnerable are health care workers and relatives who come in much closer contact with the sick. 
 
 Still, Liberia is among the poorest countries in the world, and the outbreak already has taxed the capacity of local health clinics and doctors, prompting concern for those who remain in the country. 
 
 In a statement released Wednesday, the Peace Corps said that 340 volunteers in the three affected countries were being evacuated and "a determination on when volunteers can return will be made at a later date." 
 
 Two U.S.-based missionary groups also have ordered the evacuation of their non-essential personnel from Liberia after a doctor and a missionary both contracted Ebola. 
 
 SIM USA President Bruce Johnson announced Tuesday that his group and Samaritan's Purse decided on the evacuation following an upsurge in the number of Ebola cases in Liberia. A Texas-trained doctor and a missionary from North Carolina have contracted the disease and are in isolation in Liberia. 
 
 ___ 
 
 Associated Press writer Krista Larson in Dakar, Senegal contributed to this report. ||||| Media playback is unsupported on your device Media caption The current outbreak is the deadliest since Ebola was discovered in 1976 
 
 Liberia's government has announced that it is closing down all schools across the country to stop the spread of the deadly Ebola virus. 
 
 Some communities would be placed under quarantine as well, President Ellen Johnson Sirleaf said. 
 
 Non-essential government workers will be sent home for 30 days and the army deployed to enforce the measures. 
 
 The number of people killed by the virus in West Africa has now reached 672, according to new UN figures. 
 
 The BBC's West Africa correspondent Thomas Fessy says treatment facilities have reportedly been overwhelmed in the Liberian capital Monrovia. 
 
 Ebola virus disease (EVD) 
 
 Image copyright Science Photo Library 
 
 Symptoms include high fever, bleeding and central nervous system damage 
 
 Fatality rate can reach 90% 
 
 Incubation period is two to 21 days 
 
 There is no vaccine or cure 
 
 Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery 
 
 Fruit bats are considered to be virus' natural host 
 
 Profile: Dr Sheik Umar Khan 
 
 Some wards have already filled up, forcing health workers to treat some patients at their homes. 
 
 President Sirleaf said that Friday 1 August would be a non-working day in Liberia to allow for the disinfection of all public facilities. 
 
 "All non-essential staff - to be determined by the heads of ministries and agencies - are to be placed on 30 days' compulsory leave," she added. 
 
 Rapid spread 
 
 The US humanitarian organisation Peace Corps said it was withdrawing 340 volunteers from Liberia, Sierra Leone and Guinea because of the spread of the virus. 
 
 Two of its volunteers had been isolated after being exposed to someone who later died from the virus, it added. 
 
 Media playback is unsupported on your device Media caption Dr Stephan Monroe: "Outbreak presents challenges to do what is needed to break the chains of transmission" 
 
 A Peace Corps spokeswoman said: "These volunteers are not symptomatic and are currently isolated and under observation." 
 
 Ebola kills up to 90% of those infected, but patients have a better chance of survival if they receive early treatment. 
 
 It spreads through contact with an infected person's bodily fluids. 
 
 The outbreak - the world's deadliest to date - was first reported in Guinea in February. It then spread to Liberia and Sierra Leone.
– The Peace Corps is evacuating its volunteers from the three nations in West Africa hit hard by the Ebola epidemic, reports CNN. The 340 Americans will be returning to the US from Liberia, Sierra Leone, and Guinea as the outbreak worsens. In fact, two Peace Corps workers must remain behind in isolation because they had contact with an infected person, though neither has shown symptoms, says the group. The move comes as health officials try to contain the disease to West Africa. Today, Liberia shut down all its schools, sent home non-essential government workers for three weeks, and dispatched troops to enforce quarantines in some communities, reports the BBC. "My fellow Liberians, Ebola is real, Ebola is contagious and Ebola kills," said President Ellen Johnson Sirleaf. "Denying that the disease exists is not doing your part, so keep yourselves and your loved ones safe." With 670 deaths and counting, the outbreak is now the largest in history, reports the AP. "This epidemic is without precedent," says the director of Doctors Without Borders. "It's absolutely not under control, and the situation keeps worsening."