9 of the Worst Viruses on Earth

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Going viral

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You've heard of Ebola and likely Zika, but there are many dangerous viruses that have yet to make horrific headlines. A panel of scientists and public health experts gathered by the World Health Organization (WHO) met to discuss the top emerging pathogens most likely to cause severe outbreaks in the near future. These are viruses for which there is no vaccine and few, if any, treatments.

Here's a look at these diseases and what makes them so frightening.

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​Crimean-Congo hemorrhagic fever

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Spread primarily to people from ticks and livestock, the Crimean-Congo hemorrhagic fever first emerged in Crimea in 1944, where it got the name Crimean hemorrhagic fever, according to the Centers for Disease Control and Prevention (CDC). Later, when it was recognized in 1969 as the culprit for illness in the Congo, the name changed to reflect the resulting illness. CCHF outbreaks have a fatality rate of as high as 40%, according to the WHO, and there is no vaccine.

Symptoms come on suddenly and include headache, high fever, vomiting, and back, joint and stomach pain. As the illness continues, there can be severe bruising and nosebleeds, as well as bleeding in the face, mouth and throat.

In some cases, the virus can be transmitted between people due to close contact with infected blood or bodily fluids. CCHF is found in many places including Africa, central Asia, the Middle East, eastern and southern Europe and India.

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Ebola virus disease

Photo: CDC [public domain}/Wikimedia Commons

The first known cases of Ebola, also known as Ebola hemorrhagic fever, were in 1976. Although early outbreaks occurred in remote villages in Central Africa near tropical rain forests, reports the WHO, more recent outbreaks were in urban areas as well. This severe illness results from a virus that is transmitted to people from wild animals then spread from humans to humans. The average fatality rate is 50% but has been as high as 90% in some outbreaks.

There are no Ebola vaccines but there are clinical trials under evaluation. Symptoms range from fever and vomiting to bleeding and weakness. Recovery depends on good supportive care and the patient's immune system.

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Marburg hemorrhagic fever

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Two mysterious outbreaks that occurred in Europe in the '60s were traced back to laboratory workers who handled African monkeys that had been imported from Uganda. A filovirus from the Ebola family of viruses, the virus is named after Marburg, the city in Germany where it was first detected. The natural host for the virus is believed to be fruit bats of the Pteropodidae family. The virus is transmitted from the bats to people and then spread through human-to-human contact.

Symptoms, which include high fever, severe headache and muscle aches, begin abruptly and quickly progress to include gastrointestinal issues, extreme lethargy and bleeding. In fatal cases — which up to 88% of instances are — death occurs in less than 10 days after symptoms start. There is no vaccine or treatment available.

The WHO has reported outbreaks in Angola, Democratic Republic of the Congo, Kenya, South Africa and Uganda.

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Lassa fever

Photo: CDC [public domain]

About 80% of people who become infected with the Lassa virus never have symptoms, reports the WHO. That's what makes it so difficult to detect the virus. The virus is a zoonotic illness that is transmitted to people via contact with food or household items contaminated with rat urine or feces. But it can also be spread from person to person via infected blood or fluids.

For those who have symptoms, they are usually mild and include a low fever and overall weakness. Those who have more serious symptoms may experience bleeding in their eyes, gums and nose, as well as vomiting, respiratory distress, facial swelling, deafness and severe pain. Only about 1% of Lassa virus infections result in death.

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MERS and SARS coronavirus diseases

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Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS) are part of the coronavirus family — viruses that usually cause upper respiratory illnesses. Though virus transmission seems to come from infected camels, reports Smithsonian, both illnesses are easily spread through human sneezes and coughs.

SARS was first reported in Asia in 2003, according to the CDC, but the global outbreak was quickly contained and no more cases have been reported since 2004. MERS, however, was first reported in 2012 and has since spread to other countries. About 36% of patients who have reported MERS (shown here) have died, according to the WHO. There's no vaccine or treatment currently available.

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Nipah and Rift Valley fever

Photo: CDC [public domain]/Wikimedia Commons

The WHO recently named both these zoonotic diseases as some of the most dangerous emerging pathogens likely to cause severe outbreaks with a fatality rate as high as 75%. Nipah virus infection was first identified in 1998 when pig farmers in Malaysia became ill. More than one million pigs were euthanized as a response. In later outbreaks in India and Bagladesh, there were no obvious hosts. In 2018, the virus reappeared in India and killed 17. The virus was traced back to infected fruit bats (a natural host for the virus) in a well and rabbits being bred, reports CNN. Symptoms include respiratory issues and mental confusion.

Rift Valley fever was first identified in 1931 in sheep farmers in Kenya and has since been found in outbreaks mostly throughout Africa. The disease is transmitted by handling infected animal tissue, drinking infected milk or through the bites of infected mosquitoes. There has been no documented case of human-to-human transmission. Symptoms include fever, muscle pain and headache. A small percentage of patients get ocular disease or brain inflammation.

Neither Nipah virus infection or Rift Valley fever have vaccines.

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Chikungunya

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Chikungunya (pronounced chick-un-goon-ya) virus is spread to people by Aedes aegypti and Ae. albopictus (commonly known as the Asian tiger mosquito), the same insects that introduced us to dengue fever. Identified by the WHO as "serious," it causes fever and severe, sometimes debilitating joint pain. Other symptoms include muscle pain, headache, nausea, fatigue and rash. Most patients eventually recover, but often joint pain can persist for months and even years.

Disease outbreaks have occurred in Africa, Asia, Europe, and the Indian and Pacific Oceans, reports the CDC. In late 2013, the chikungunya virus was found for the first time in the Americas on islands in the Caribbean and has since been reported in the U.S. There is no vaccine or treatment.

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Severe fever with thrombocytopenia syndrome

Photo: Erhabor Osaro [CC BY-SA 3.0]/Wikimedia Commons

Also identified by the WHO as "serious," severe fever with thrombocytopenia syndrome (also known as thrombocytopenia) is an emerging disease found in China, Japan and South Korea. The virus appears to be spread by ticks, reports UpToDate.

The main symptom is a very high fever, as well as a drop in blood platelets that causes tissue bleeding, bruising and slow blood clotting. According to a recent CDC study, the fatality rate for SFTS in South Korea was 47.2%. Currently there is no treatment or vaccine available.

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Zika

Photo: CDC [public domain]

This once-obscure virus has been wreaking havoc recently, especially in the Americas. The WHO declared Zika a global public health emergency, especially because of newfound links with birth defects and other potentially severe health problems.

Zika is transmitted by infected Aedes mosquitoes like Aedes aegypti and Aedes albopictus, also known as the Asian tiger mosquito. Although only about one in five people infected with Zika virus will get sick, according to the CDC, the virus is linked to microcephaly, a birth defect characterized by an undersized head and incomplete brain development.