MERS, SARS, Ebola: How dangerous are they?

An infectious disease with an exotic name has again captured public attention.

This time, the virus isn't Ebola, but MERS, or Middle East Respiratory Syndrome.

South Korea has quarantined 1,369 people and has closed hundreds of schools in an effort to control the disease. The outbreak has now infected 35 people, said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

The outbreak could get much larger, given the number of people exposed at hospitals and health clinics by patients who were infectious, but not yet diagnosed, according to the World Health Organization.

MERS is one of many diseases that scientists call emerging infections, or new diseases with the potential to become more common in the near future.

Some of the panic about Ebola last year stemmed from the public's confusion about what emerging viruses are and how they spread, said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. USA TODAY asked infectious disease experts to explain some of the key similarities and differences among these infections.

MERS

The MERS virus, which belongs to the coronavirus family, was identified 2012 in Saudi Arabia. Since then, it's infected at least 1,179 people and has killed 442, according to WHO. MERS causes fever, coughing, difficulty breathing and, in some cases, pneumonia and kidney failure, according to the Centers for Disease Control and Prevention.

How does it spread?

Scientists don't yet understand exactly how people spread MERS, Hotez said. In the past, the virus has spread to people in close contact with patients, such as family members living together. Yet in the South Korean outbreak, some people were infected after being exposed to an infectious patient for as little as five minutes.

SARS

SARS, a coronavirus also known as Severe Acute Respiratory Syndrome, first appeared in 2002 and was identified in 2003. SARS infected nearly 8,100 people, killing about 10%, according to the WHO. It disappeared after 2004, for reasons that scientists don't totally understand. SARS begins with a high fever and flu-like symptoms, including headache, body aches and cough, and usually progresses to pneumonia, according to the CDC.

How does it spread?

SARS appears to spread through close contact, most likely through respiratory droplets that fly into the air as people sneeze or cough, Hotez said. These droplets normally don't travel more than three feet before falling to the ground. Droplets can infect people in two ways -- either by directly landing in someone's nose, mouth or eyes, or by landing on a surface that someone touches.

Ebola

Ebola emerged in 1976 in Zaire. In the West African outbreak, which began in December 2013, more than 27,000 people have been infected and more than 11,000 have died, according to the WHO. Over the years, mortality rates have ranged from 25% to 90%. Ebola causes a fever, headache, vomiting, diarrhea and, in some cases, bleeding, according to the CDC.

Although Liberia was recently declared Ebola-free, the virus continues to simmer in Guinea and Sierra Leone. Some infectious disease experts worry that Ebola could become endemic in Guinea, spreading indefinitely and routinely, much like malaria, said Daniel Lucey, an adjunct professor of microbiology and immunology at Georgetown University Medical Center.

How does it spread?

Ebola spreads through direct contact with bodily fluids, such as blood, according to the CDC. It does not spread through the air. Patients are not contagious until they develop symptoms. They are most contagious in the advanced stages of the disease, when they are likely to be hospitalized, which puts health care workers at high risk.

One thing that makes diseases such as MERS so frightening is the lack of proven treatments and vaccines. Hotez said he's frustrated by the lack of drug development, given that the world has had years to prepare for these diseases.

Although researchers are testing vaccines against Ebola in people in West Africa, none are approved. Hotez and colleagues at Baylor have developed an experimental vaccine against SARS, but it's never been tested on people.

There is also no vaccine against H7N9 bird flu, which emerged in 2013 and which has killed 212 of 571 people infected, according to the WHO.

Scientists have developed licensed vaccines for just two of the emerging viruses: H1N1 influenza, which emerged in 2009, and H5N1 bird flu, which emerged in 1997 and has killed 447 of 840 people infected.

The flu shots used this year protected against H1N1.

The vaccine against H5N1 could be used in the event of an outbreak. While people have been infected with H5N1 and H7N9 through exposure to infected poultry, neither of these viruses spreads easily from person to person, which has so far limited the extent of outbreaks.

The biggest hurdles to creating new vaccines are not scientific, but financial, Hotez said.

Scientists laid the groundwork for an effective Ebola vaccine years ago, but until the disease broke out in West Africa last year, drug companies had little interest in developing the vaccines.

The South Korean outbreak could provide momentum for new research on MERS. The country's experience suggests MERS may have the potential to become a pandemic, said Shibo Jiang, director of the Institute of Medical Microbiology at Fudan University in Shanghai.

Until now, scientists thought MERS had little potential to spread widely because infections seemed to require very close contact. Many patients never infected anyone. Others infected only their care givers, such as hospital nurses or relatives living in the same home. In South Korea, however, the first patient infected 26 people.