The 13 Scariest Viruses on Earth Today  

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They say these guys have existed since the beginning of life on Earth. As far back as there's a plant or animal record, there are viruses. Life has adapted, changed, grown... and still the virus plagues it. It has been re-inventing itself over and over again in order to outwit, outlast and outplay its victims. They started as humble messengers, genetic strands that carried hereditary information from newly developed life to its offspring. As life transformed and become more complex, viruses lost their primary function when cells took over their role.

But the virus didn't take so well to being fired, and like any disgruntled ex-employee, began a millennia-spanning march to destroy that which it no longer served. They became unstoppable parasites, infecting rather than exchanging genes with their hosts; proscribing each cell with their own genetic formula. They developed the ability to jump from species to species by changing their genetic material to fit the host they infected.

The virus of today is highly complex and nearly impossible to control or contain. Over the past million+ years, they've developed a level of survivalism and efficiency that is astounding to behold... even to comprehend... even as we suffer from their success, its impossible not to admire them. We keep studying, keep trying to find new ways to defeat them, and they continue to calmly mutate around every new thing we throw at them.

Alive or dead? So far there has been no known form of life on Earth that is not susceptible to deadly viruses. They are small enough to hide between light waves, too small to be seen by anything but an electron microscope. It can lie dormant for long periods of time, indefinitely in some cases. It is not technically a life form, but you can ask the question: is the virus the most successful organism in the history of our planet?

We have been struggling with the deadliest, worst viruses since the beginning of humanity itself. Whether you've suffered from the common cold or something more rare and gut wrenching, you've definitely come into contact with a human virus. The names of viruses on this list should strike fear into your very heart. Just thank the starts that you've never caught on of these dangerous, deadly strains.

(PS. Before you head to the comments section in a huff, the Black Death was not a virus, it was bacterial.)
Rotavirus is listed (or ranked) 1 on the list The 13 Scariest Viruses on Earth Today
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61,000 Lives a Year

According to the WHO, this merciless virus causes the deaths of more than half a million children every year. In fact, by the age of five, virtually every child on the planet has been infected with the virus at least once. Immunity builds up with each infection, so subsequent infections are milder. However, in areas where adequate healthcare is limited the disease is often fatal. Rotavirus infection usually occurs through ingestion of contaminated stool. Because the virus is able to live a long time outside of the host, transmission can occur through ingestion of contaminated food or water, or by coming into direct contact with contaminated surfaces, then putting hands in the mouth.
Once it’s made its way in, the rotavirus infects the cells that line the small intestine and multiplies. It emits an enterotoxin, which gives rise to gastroenteritis. see more on Rotavirus
Ebola virus disease is listed (or ranked) 2 on the list The 13 Scariest Viruses on Earth Today
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Typically less than 100 lives a year. UPDATE: A severe Ebola outbreak was detected in West Africa in March 2014. The number of deaths in this latest outbreak has outnumbered all other known cases from previous outbreaks combined. The World Health Organization is reporting nearly 2,000 deaths in this latest outbreak. 

Once a person is infected with the virus, the disease has an incubation period of 2-21 days; however, some infected persons are asymptomatic. Initial symptoms are sudden malaise, headache, and muscle pain, progressing to high fever, vomiting, severe hemorrhaging (internally and out of the eyes and mouth) and in 50%-90% of patients, death, usually within days. The likelihood of death is governed by the virulence of the particular Ebola strain involved. Ebola virus is transmitted in body fluids and secretions; there is no evidence of transmission by casual contact.

There is no vaccine and no cure.

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HIV is listed (or ranked) 3 on the list The 13 Scariest Viruses on Earth Today
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3.1 Million Lives a Year

Human Immunodeficiency Virus has claimed the lives of more than 25 million people since 1981. HIV gets to the immune system by infecting important cells, including helper cells called CD4+ T cells, plus macrophanges and dendritic cells. Once the virus has taken hold, it systematically kills these cells, damaging the infected person’s immunity and leaving them more at risk from infections. The majority of people infected with HIV go on to develop AIDS. Once a patient has AIDS common infections and tumours normally controlled by the CD4+ T cells start to affect the person.

In the latter stages of the disease, pneumonia and various types of herpes can infect the patient and cause death.

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Smallpox is listed (or ranked) 4 on the list The 13 Scariest Viruses on Earth Today
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Officially eradicated - Due to it's long history, it impossible to estimate the carnage over the millennia

Smallpox localizes in small blood vessels of the skin and in the mouth and throat. In the skin, this results in a characteristic maculopapular rash, and later, raised fluid-filled blisters. It has an overall mortality rate of 30–35%. Smallpox is believed to have emerged in human populations about 10,000 BC. The disease killed an estimated 400,000 Europeans per year during the closing years of the 18th century (including five reigning monarchs), and was responsible for a third of all blindness. Of all those infected, 20–60%—and over 80% of infected children—died from the disease.

Smallpox was responsible for an estimated 300–500 million deaths during the 20th century alone. In the early 1950s an estimated 50 million cases of smallpox occurred in the world each year. As recently as 1967, the World Health Organization (WHO) estimated that 15 million people contracted the disease and that two million died in that year. After successful vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the eradication of smallpox in December 1979.

Smallpox is one of only two infectious diseases to have been eradicated by humans, the other being Rinderpest, which was unofficially declared eradicated in October 2010.

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