The outbreak of Ebola in West Africa is now the deadliest on record. More than 3,000 people have died and more deaths are expected.
The Centers for Disease Control and Prevention confirmed the first case of Ebola in the to U.S. and has issued its highest level alert to try and control it.
"Ebola is a frightening disease, and in some respects more so than HIV because it has a high mortality rate and kills within days of infection," said John Palisano, a professor of biology at Sewanee the University of the South in Tennessee.
But what exactly is Ebola, and how has it become so deadly this time around? Here are some answers.
The World Health Organization (WHO) defines the Ebola virus as a severe disease "that's often fatal in humans."
In fact, WHO states the mortality rate is up to 90 percent in some instances.
Symptoms include the sudden onset of fever, intense weakness, muscle pain, headache and sore throat.
This is often followed by vomiting, diarrhea, rash, impaired kidney and liver function, and in some cases, both internal and external bleeding.
But experts say diagnosing Ebola can be difficult because the symptoms often mirror other diseases such as malaria, typhoid fever, cholera, plague, rickettsiosis, relapsing fever, meningitis and hepatitis.
The incubation period for Ebola—the time from infection with the virus to onset of symptoms—is two to 21 days.
And people are infectious as long as their blood and secretions contain the virus. Ebola virus infections are usually diagnosed through laboratory tests.
Ebola is first transmitted from animals to humans.
In Africa, infection has occurred through the handling of infected chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines found ill or dead.
According to WHO, humans can get the virus by touching or consuming fluids from infected animals that have been slaughtered. For example, people who eat or handle certain "bush meat" like fruit bats are at particular risk.
Humans in turn transmit the virus among themselves from direct contact through broken skin or mucous membranes with the blood, or other bodily fluids or secretions (stool, urine, saliva, semen, vomit) of infected people.
For example, health-care workers have frequently been infected while treating patients with Ebola. They can get it by exposure to objects, such as needles or changing soiled bed linens, which have been contaminated with infected secretions.
But this happens when infection control precautions are not strictly practiced.
Burying the dead has also been a way the virus has been spread. Bodies of the dead can be even more contagious than living Ebola patients. In some areas in West Africa, burial traditions where the family prepares the body conflict with WHO's safety measures.
The first recorded outbreak of Ebola was in 1976 in Zaire, according to the CDC. There were 380 cases that resulted in 280 deaths—a mortality rate of 88 percent.
That same year, an outbreak in Sudan killed 151 people among 284 reported cases.
Nearly 30 other outbreaks of Ebola have occurred since then, with the latest before this most recent instance in Uganda between 2012 and 2013.
Most outbreaks have been in Africa, with some limited cases during the last two decades reported in England, the Philippines, Russia and Italy.
Ebola was introduced in the U.S. when it was found in quarantined monkeys imported from the Philippines in 1989, 1990 and 1996.
But no human infections were discovered at that time.
Recently in the U.S., two American aid workers were diagnosed with Ebola in Liberia and treated in Atlanta.
It's believed the outbreak started in Liberia, then spread to Guinea and Sierra Leone. But there are now cases in Nigeria, Africa's most populous country, where the government has declared a state of emergency over the virus.
Analysts say this latest outbreak is potentially more lethal because this current strain of the virus in more virulent than others.
The recent outbreak also has spread quickly to urban areas. Past outbreaks were more confined to rural areas, and though deadly, burned themselves out.
Also helping to spread the current virus are burial practices, which include washing the bodies of victims that still carry the virus. And relatives of virus carriers have been hiding them from medical aid to pursue home remedies.
News reports cite incidents where some people in Sierra Leone don't believe the Ebola virus is real and a threat to human health. That's despite a state of emergency by the government there, and strict enforcement of quarantine measures.
There's no current licensed vaccine to stop the Ebola virus, according to WHO. But there are some experimental treatments.
The two U.S. aid workers now being treated at Emory have been given Zmapp. The treatment is a mixture of three antibodies against the Ebola virus, produced in bioengineered tobacco plants.
Another experimental drug, developed by the Canadian firm Tekmira Pharmaceuticals has been tested on monkeys and on some non-Ebola infected humans.
The drug is designed to target the strands of genetic material of the virus (RNA). An early safety trial has been put on hold because the Food and Drug Administration has requested more data.
And the American based pharmaceutical company Sarepta Therapeutics is working on a similar treatment. It has been tested in healthy human volunteers, but not on an Ebola patient.
But for now, treatment is usually strict isolation of patients, while giving them intravenous fluids to stop dehydration along with antibiotics to fight infections.
Most experts say the U.S. is immune from a deadly outbreak of Ebola, similar to what's happening in West Africa.
That's because the U.S. has a more highly refined and sophisticated medical system that would catch the disease before it could spread, the experts say.
But that sentiment comes with a warning.
"Americans should still be on the alert," said Cecilia Rokusek, assistant dean for education, planning and research at Nova Southeastern University in Florida.
"We can no longer become complacent to any public health danger anywhere," she said.
—By CNBC's Mark Koba