The Ebola virus has now killed more than 1,100 people in West Africa. Although the mortality rate of the most recent outbreak isn’t as high as in previous events, it’s still the case that most people who become infected with Ebola will not survive. (The mortality rate is about 60 percent for the current outbreak, compared with 90 percent in the past, according to the National Institutes of Health.)
But while the Ebola virus is certainly terrifying, it’s not the most dangerous virus in the world. That honor goes to…
1. Marburg Virus
First identified in 1967, the Marburg virus is often considered the most dangerous disease in the world. It is named after a small and idyllic town on the river Lahn – but that has nothing to do with the disease itself, which causes severe illness and usually death in a short amount of time. The Marburg virus is a hemorrhagic fever virus; as with Ebola, the Marburg virus causes terrifying symptoms including convulsions and bleeding of mucous membranes, skin, and organs. Also like Ebola, Marburg is transmitted by direct contact with the blood, body fluids, and tissues of infected persons. There is no treatment for the virus, which has a fatality rate of around 90 percent.
2. Ebola Virus
There are five strains of the Ebola virus, each named after the countries and regions where they were first discovered: Zaire, Sudan, Tai Forest, Bundibugyo, and Reston. The Zaire Ebola virus is the deadliest, with a mortality rate of up to 90 percent. It is the strain currently spreading through Guinea, Sierra Leone and Liberia, and beyond. Scientists say bats transmit the Zaire Ebola virus into the human population, though there may be other animal hosts, as well. While there is currently no approved treatment for Ebola, scientists are working on several new drugs to treat the virus and the National Institutes of Health will begin testing an Ebola vaccine on humans this fall.
Hantavirus actually describes several types of viruses, many of which are harmful to humans. It is named after a river where American soldiers were first thought to have been infected with the Hantavirus, during the Korean War in 1950. Infection with hantavirus can progress to Hantavirus Pulmonary Syndrome (HPS), which can be fatal; case fatality rates vary, with about one-third to one-half of cases resulting in death. Symptoms include lung disease, fever and kidney failure. What makes hantavirus particularly dangerous is its ubiquitous animal host: the common field mouse, found across the world and in nearly every state in the US. The rodents shed the virus in their urine, droppings, and saliva. Hantavirus is an airborne virus, primarily transmitted to people when they breathe in air contaminated with the virus. For that reason, you should always wear a facial mask if you’re cleaning out an attic or other room where mouse droppings are present. Cases of HPS have been reported in 34 states, though it’s far more common west of the Mississippi River.
4. Avian (bird) Flu
The various strains of Avian flu (also called bird flu) regularly cause panic – which is perhaps justified because the mortality rate for those infected with the virus is 70 percent. But in fact the risk of contracting the H5N1 strain – one of the best known – is quite low. You can only be infected through direct contact with live or dead poultry; there is no evidence that the disease can be spread to people through properly cooked food. Since 2003, 650 human infections of H5N1 viruses have been reported to the World Health Orgranization (WHO) by 15 countries. Six countries— Bangladesh, China, Egypt, India, Indonesia, and Vietnam—have widespread and ongoing infections in their poultry. Because of the high rate of infection in poultry and the severity of infection in humans, the WHO considers H5N1 to be a highly pathogenic virus with strong ‘pandemic potential.’
5. Lassa Virus
A nurse in Nigeria was the first person to be infected with the Lassa virus, a zoonotic, or animal-borne, virus primarily transmitted by rodents. The virus is endemic in Western Africa, where scientists estimate that 15 percent of rodents carry the virus. Person-to-person infections and laboratory transmission can also occur, particularly in the hospital environment in the absence of adequate infection control measures. Infection with the virus causes Lassa fever, an acute hemorrhagic illness similar to Ebola. The case fatality rate of Lassa fever varies, but generally the virus kills about half of those it infects. One of the scariest aspects of the virus is its high rate of infection. The number of Lassa virus infections per year in west Africa is estimated at 100,000 to 300,000; in some areas of Sierra Leone and Liberia, it is known that up to one in six people admitted to hospitals every year have Lassa fever, which indicates the serious impact of the disease on the population of this region.
6. Junin Virus
The Junin virus is a type of arenavirus that causes Argentine hemorrhagic fever. Early symptoms of infection include chills, malaise, headache, and muscle pain. Early identification and treatment are key; however, the symptoms can be so indistinct that the disease is rarely detected in the first instance. Without medical intervention, infected persons will progress to the neurological-hemorrhagic stage of the disease, at which point symptoms become far more severe. The virus kills about one-third of those infected with it, but unlike many of the other infections on this list, Argentine hemorrhagic fever can be prevented with a vaccine known as Candid #1.
7. Crimean-Congo Hemorrhagic Fever
Crimean-Congo hemorrhagic fever (CCHF) is caused by infection with a tick-borne virus (Nairovirus). The disease was first characterized in the Crimea in 1944 and given the name Crimean hemorrhagic fever. It was then later recognized in 1969 as the cause of illness in the Congo, thus resulting in the current name of the disease. The virus is similar to the Ebola and Marburg viruses in the way it progresses. During the first days of infection, sufferers present with pin-sized bleeding in the face, mouth and the pharynx. Like other hemorrhagic fevers, CCHF has a high case fatality rate, resulting in the deaths of nearly half of those infected with the virus. Crimean-Congo hemorrhagic fever is incredibly widespread, with documented cases in Eastern Europe (particularly in the former Soviet Union), throughout the Mediterranean, in northwestern China, central Asia, southern Europe, Africa, the Middle East, and the Indian subcontinent.
8. Machupo Virus
First described in 1959, the Machupo virus is an arenavirus that causes Bolivian hemorrhagic fever. Locals, who witnessed the effects of the virus on the farmers it first infected in the 1950’s and 1960’s, often referred to it as “Black Typhus,” alluding to the hemorrhaging, high fever, pain, and rapid death caused by the illness. The progression of the Machupo virus is similar to that of the Junin virus, as is its case fatality rate (about 30 percent). Death can occur between a few hours and a few days after the onset of symptoms. The common field mouse is the animal host of Machupo virus; infection occurs through aerosolized, food-borne, or direct contact with virus particles. The virus can also be transmitted from human to human.
9. Kyasanur Forest Virus
Scientists discovered the Kyasanur Forest Virus in woodlands on the southwestern coast of India in 1955. It is transmitted by ticks, but scientists say it is difficult to determine its carriers. It is assumed that rats, birds and boars could be hosts. People infected with the virus suffer from high fever, strong headaches, muscle pain, and bleeding. While the case fatality rate of Kyasanur Forest Disease (under 10 percent) is far lower than many of the other viruses on this list, its the second stage of symptoms that make this such a scary disease. After 1-2 weeks of symptoms, some patients recover without complication; however, the disease is biphasic for a subset of patients (about one in five) who experience a second wave of symptoms at the beginning of the third week. These symptoms include fever and malaise, as well as severe neurological manifestations including mental disturbances, hallucinations and delusions, tremors, and vision deficits.
10. Dengue Fever
Dengue fever is a constant threat: with nearly half of the world’s population living in areas at risk for infection, the mosquito-borne virus is a leading cause of illness and death in the tropics and subtropics. The global incidence of dengue fever has risen dramatically in recent years, with as many as 400 million people infected with the virus in 2010. Along with the rise in the overall number of cases, the geographical distribution of dengue fever has also undergone marked changes in recent decades. In 2009, the US, which had not seen a case of the virus since the early 1900’s, suddenly found themselves dealing with an outbreak of at least 22 locally-acquired infections in Key West, Florida. Furthermore, as the WHO reports, “[n]ot only is the number of cases increasing as the disease spreads to new areas, but explosive outbreaks are occurring” involving rapid transmission in the population. There is no specific treatment for dengue fever, which causes symptoms such as severe abdominal pain, persistent vomiting, rapid breathing, bleeding gums, fatigue, restlessness, and blood in vomit. The virus kills up to one in five of those infected, but with adequate medical care the fatality rate drops as low as 1 percent.