WASHINGTON (AP) The United States' top disease detective calls Ebola a "painful, dreadful, merciless virus."
The World Health Organization has declared the outbreak in West Africa an international emergency, killing more than 900 people and spreading.
That's scary and serious. But it also cries out for context.
AIDS alone takes more than a million lives per year in Africa a thousand times the toll of this Ebola outbreak so far.
Lung infections such as pneumonia are close behind as the No. 2 killer. Malaria and diarrhea claim hundreds of thousands of African children each year.
In the United States, where heart attacks and cancer are the biggest killers, the risk of contracting the Ebola virus is close to zero.
Americans fretting about their own health would be better off focusing on getting a flu shot this fall. Flu is blamed for about 24,000 U.S. deaths per year.
To put the Ebola threat in perspective, here are some reasons to be concerned about the outbreak, and reasons not to fear it:
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WHY IT'S SCARY
There is no cure for Ebola hemorrhagic fever.
More than half of people infected in this outbreak have died. Death rates in some past outbreaks reached 90 percent.
It's a cruel end that comes within days. Patients grow feverish and weak, suffering through body aches, vomiting, diarrhea and internal bleeding, sometimes bleeding from the nose and ears.
The damage can spiral far beyond the patients themselves.
Because it's spread through direct contact with the bodily fluids of sick patients, Ebola takes an especially harsh toll on doctors and nurses, already in short supply in areas of Africa hit by the disease.
Outbreaks spark fear and panic.
Health workers and clinics have come under attack from residents, who sometimes blame foreign doctors for the deaths. People with Ebola or other illnesses may fear going to a hospital, or may be shunned by friends and neighbors.
Two of the worst-hit countries Liberia and Sierra Leone sent troops to quarantine areas with Ebola cases. The aim was to stop the disease's spread but the action also created hardship for many residents.
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WHERE IT IS
The outbreak began in Guinea in March before spreading to neighboring Sierra Leone and Liberia. A traveler recently carried it farther, to Nigeria, leading to a few cases in the giant city of Lagos.
Ebola emerged in 1976. It has been confirmed in 10 African nations, but never before in the region of West Africa.
Lack of experience with the disease there has contributed to its spread. So has a shortage of medical personnel and supplies, widespread poverty, and political instability.
Sierra Leone still is recovering from a decade of civil war in which children were forced into fighting. Liberia, originally founded by freed American slaves, also endured civil war in the 1990s. Guinea is trying to establish a young and fragile democracy.
Nigeria, Africa's most populous country, boasts great oil wealth but most of its people are poor. The government is battling Islamic militants in the north who have killed thousands of people and kidnapped more than 200 schoolgirls in April.
This outbreak has proved more difficult to control than previous ones because the disease is crossing national borders, and is spreading in more urban areas.
Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, predicts that within a few weeks, Ebola will sicken more people than all previous occurrences combined. Already more than 1,700 cases have been reported.
Global health officials say it will take months to fully contain the outbreak, even if all goes as well as can be hoped.
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REASONS NOT TO BE AFRAID
Ebola is devastating for those it affects. But most people don't need to fear it. Why?
Ebola doesn't spread easily, the way a cold virus or the flu does. It is only spread by direct contact with bodily fluids such as blood, saliva, sweat and urine. Family members have contracted it by caring for their relatives or handling an infected body as part of burial practices. People aren't contagious until they show symptoms, Frieden said. Symptoms may not appear until 21 days after exposure.
"People should not be afraid of casual exposure on a subway or an airplane," said Dr. Robert Black, professor of international health at Johns Hopkins University.
Health officials around the developed world know how to stop Ebola. Frieden described tried-and-true measures: find and isolate all possible patients, track down people they may have exposed, and ensure strict infection-control procedures while caring for patients. Every past outbreak of Ebola has been brought under control.
The CDC is sending at least 50 staff members to West Africa to help fight the disease, while more than 200 work on the problem from the agency's headquarters in Atlanta. The WHO is urging nations worldwide to send money and resources to help.
It's true that Ebola could be carried into the United States by a traveler, possibly putting family members or health care workers at risk. It's never happened before. But if the disease does show up in the U.S., Frieden said, doctors and hospitals know how to contain it quickly.
"We are confident that a large Ebola outbreak in the United States will not occur," Frieden told a congressional hearing Thursday.
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OTHER THINGS TO WORRY ABOUT
Ebola's toll is minuscule compared with other diseases that kill millions of people.
"The difference is the diseases that do kill a lot of people malaria, diarrhea, pneumonia they cause their problems over time," Black said. "They're not generally epidemic. They're not the kind of sudden burst of disease and death that creates fear like this."
The common diseases have far lower mortality rates. They kill so many people because such huge numbers are infected.
In comparison, Ebola is manageable.
"The order of magnitude of the resources to control Ebola in small communities in three or four countries is very small compared to controlling malaria in all of Asia and Africa," Black said. "I don't at all think we should hold back on the resources to control Ebola, but we need more resources to control these major killers of children and adults that we're making too little effort against."
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Associated Press writers Lauran Neergaard in Washington, Marcia Chen in London, and Michael Stobbe in New York contributed to this report.
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