6 deadliest viruses of the 21st century

By Naveed Saleh, MD, MS, for MDLinx
Published September 25, 2019

Key Takeaways

Medicine has made great strides in the past several centuries, eliminating or containing devastating viruses such as smallpox and polio. But some viruses—like Ebola—continue to confront humanity as major global health threats.

Viruses come in various stripes—differing greatly in genome organization, structure, expression, replication, and transmission—and can infect humans, animals, plants, and single-celled organisms, including mycoplasmae, bacteria, and algae. Some viruses infect only one species of the host while others can infect several species. Viruses are thought to have evolved from either the DNA or RNA of host cells that developed the ability to replicate autonomously. Alternatively, viruses could be degenerate forms of intracellular parasites. Some viruses exert no effect after infecting a host cell. Others, however, can be lethal.

Let’s take a look at six of the most dangerous viruses that have put the world on high alert in the 21st century.

Marburg virus

Both the Marburg and Ebola viruses are members of the filovirus family and are characterized by hemorrhagic fever. Other symptoms include fever, headache, sore throat, myalgia, arthralgia, epigastric pain, vomiting, and diarrhea. In 1967, the Marburg virus was discovered to cause human illness by researchers in Marburg, Germany, following exposure to green monkeys endemic to Uganda. Bats are thought to be the natural reservoir for the virus, but this hypothesis has yet to be confirmed. The death rate for the Marburg virus is as high as 90%, with infected people usually dying of bleeding into the gastrointestinal tract and skin, shock, disseminated intravascular coagulation, and multiorgan failure. No antiviral therapy or other vaccine exists for the Marburg virus.

Ebola virus

We all know about the 2014–2016 Ebola virus outbreak that occurred in West Africa—the largest outbreak since the discovery of the virus in 1976. But, did you know that more than 11,000 people died during this outbreak? Scientists do not yet know where the rare yet fatal Ebola virus comes from, but they believe that it is animal-borne, based on the nature of similar viruses, with fruit bats being the most likely source. The bats carrying the virus can transmit it to other animals, such as primates, spreading it to the human population. Human-to-human exposure and transmission are also possible via direct contact with infected blood and body fluids.

Symptoms of Ebola virus can be sudden, and include fever, fatigue, muscle pain, headache, sore throat, vomiting, diarrhea, rash, impaired kidney and liver function, and internal and external bleeding. Of note, the average case fatality rate is approximately 50%, according to the World Health Organization (WHO). Because there is no proven treatment (ie, antiviral drugs) for the Ebola virus, supportive care (rehydration with oral or intravenous fluids) and treatment of specific symptoms are often used to improve survival outcomes, but some experimental treatments—including convalescent serum and antibody treatment—have been administered. Furthermore, a concerted research effort to develop a vaccine has been underway for some time.


In 1993, an outbreak of hantavirus pulmonary syndrome—caused by exposure to the Sin Nombre hantavirus—occurred in New Mexico and Arizona. This influenza-like disease is followed by acute respiratory failure. Deer mice serve as reservoirs for the virus, with exposure and transmission possible via inhalation of the deer mice’s urine or feces. Although rare, the virus may be transmitted if a rodent with the virus bites a human. Campers and hikers are, therefore, especially at risk. Fortunately, this virus is not spread via human-to-human contact. Early symptoms of infection include fatigue, fever, and muscle aches, particularly in large muscle groups; late symptoms—about 4 to 10 days following the initial phase of illness—include coughing and shortness of breath.

Because people don’t have antibodies to the virus, symptomatic infections are the norm, with mortality rates of 38%, according to the CDC. There is no treatment, cure, or vaccine for the virus. However, survival outcomes may be improved by early disease recognition and receipt of medical care, including oxygen therapy for severe respiratory illness.

Japanese encephalitis virus

This virus is the most frequent cause of epidemic viral encephalitis in several countries across Asia, with an estimated 68,000 clinical cases annually, according to the WHO. In fact, 24 countries in the WHO South-East Asia and Western Pacific regions have endemic Japanese encephalitis virus transmission, putting over 3 billion people at risk for infection.

Symptoms include fever, neck rigidity, altered consciousness, headache, tremors, incoordination, and convulsions. Not only is the death rate of this disease high (30%), but survivors (30% to 50%) experience serious neurologic or psychiatric sequelae. This disease is spread to humans by infected mosquitoes of the Culex species that live in Asian rice fields, with reservoirs including water birds and pigs. An inactivated vaccine does exist for protection against this virus, and mosquito protection is recommended among those at high risk of exposure. Because no antiviral therapy yet exists for infected patients, treatment is symptomatic.

Lassa fever virus

Discovered in 1969 in Lassa, Nigeria, this pathogen causes hemorrhagic fever and multiorgan failure. Mortality due to the virus is high (15% to 50%), with death due to vascular collapse. In areas where the virus is endemic, asymptomatic infection is common. The virus is most commonly transmitted by exposure to the urine of the infected small rodent Mastomys in food or water sources. The disease can also be spread via inhalation of air contaminated with infected rodent secretions. Specifically, airborne transmission may occur during cleaning activities such as sweeping. Human-to-human transmission is common in healthcare settings where proper personal protective equipment is not available or infrequently used.

Ribavirin can help if given early enough. Furthermore, hyperimmune serum, derived from survivors, can also be of benefit. No vaccine for this virus exists, with prevention taking the form of infection and rodent control.


Rabies holds the distinction of being nearly 100% fatal if untreated. The virus is found worldwide in more than 150 countries and territories. Rabies infections cause tens of thousands of deaths globally, mostly in Africa and Asia. The incubation period is generally 2-3 months, with initial symptoms of fever with pain and tingling, prickling, or paresthesia at the wound site. Fatal inflammation of the brain and spinal cord usually develops as the virus progresses to the central nervous system.

Humans infected with rabies may exhibit one of two forms of the disease: furious rabies or paralytic rabies. According to the WHO, individuals with furious rabies may exhibit signs of hyperactivity, excitable behavior, hydrophobia, and aerophobia, with death occurring after a few days due to cardiorespiratory arrest. Those with paralytic rabies—which accounts for 20% of all human cases—gradually become paralyzed, starting at the wound site. Following a coma, death eventually occurs.

Dog bites are the most common cause of infection, followed by bats. Immediately after a bite, rabies shots are curative. Additionally, after contact with a rabid animal, it is important to wash the wound with soap and water, as infection usually occurs upon exposure of the mucous membrane to saliva.

This list of deadly viruses is pretty scary, but there is hope that these pathogens will someday be eradicated. After all, humans eradicated smallpox in the 1970s several decades after the discovery of a vaccine for the disease. At one point, this virus had a mortality rate of 30%. 

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