Ebola is caused by a group of viruses belonging to the genus Orthoebolavirus, part of the Filoviridae family. These viruses carry their genetic instructions in a single strand of RNA, wrapped inside long, thread-like particles that look almost like tiny worms under an electron microscope. The average case fatality rate is around 50%, though past outbreaks have ranged from 25% to 90% depending on the viral species involved and the quality of medical care available.
The Virus Behind the Disease
Ebolaviruses are classified as filoviruses, named for their distinctive filament shape. Each viral particle contains a single strand of negative-sense RNA, roughly 13,000 to 21,000 genetic letters long. Unlike DNA viruses, RNA viruses like this one mutate relatively quickly, which is one reason different species within the genus can behave so differently in the human body.
There are six recognized species of Orthoebolavirus. Four of them cause disease in humans, with the Zaire species being the most lethal and the most common culprit in major outbreaks. The Sudan species has also triggered significant outbreaks with high mortality. A fifth species, Reston, can infect humans but has never caused illness in people, only in nonhuman primates. The variation in fatality rates between species, from 25% up to 90%, largely reflects which species is responsible.
Where the Virus Comes From
Ebola is a zoonotic disease, meaning it jumps from animals to people. Scientists believe African fruit bats are the natural reservoir, carrying the virus without becoming visibly sick themselves. The virus also circulates among nonhuman primates and forest antelopes, which can become infected and die from it just as humans do.
A “spillover event” is the moment an infected animal passes the virus to a person. This typically happens through hunting, butchering, or eating infected wildlife. Someone handling a dead chimpanzee or fruit bat in a forested region of Central or West Africa is the classic scenario for how an outbreak begins. Once a single person is infected, the virus can then spread from human to human.
How It Spreads Between People
Ebola does not spread through the air. It transmits through direct contact with the blood, body fluids, or tissues of an infected person. This includes sweat, saliva, urine, breast milk, semen, and feces. The virus can also spread through contact with contaminated surfaces or medical equipment, which is why healthcare workers in outbreak zones face such extreme risk.
One of the more concerning aspects of Ebola is how long the virus can linger in survivors. Even after someone recovers and their blood tests clear, the virus can persist for months in parts of the body that are partially shielded from the immune system. Semen is the most notable example: viral genetic material has been detected up to 40 months after the initial illness. Breast milk can harbor traces for over 16 months, and cerebrospinal fluid for up to 10 months. The eyes, placenta, and central nervous system are other sites where the virus can hide. This persistence means survivors need ongoing monitoring, and sexual transmission from a recovered patient remains a real, documented risk.
Incubation and Symptom Progression
After exposure, symptoms appear anywhere from 2 to 21 days later, with most people getting sick around day 8 to 10. The illness unfolds in two distinct phases.
The first phase involves what clinicians call “dry” symptoms: fever, headache, muscle pain, and fatigue. These look nearly identical to malaria, the flu, or dozens of other tropical infections, which makes early diagnosis difficult. After four to five days, the disease can progress to “wet” symptoms, including vomiting, diarrhea, and in severe cases, internal and external bleeding. It’s during this second phase that the virus is most contagious, because the patient is producing large volumes of infectious body fluids. Death, when it occurs, typically results from severe dehydration, organ failure, or shock caused by the body’s overwhelming inflammatory response.
Vaccines and Treatment
For decades, there was no approved vaccine or specific treatment for Ebola. That changed with the approval of ERVEBO, a single-dose vaccine now cleared by the U.S. Food and Drug Administration for anyone 12 months and older. It protects specifically against the Zaire species, which has been responsible for the largest and deadliest outbreaks. ERVEBO uses a weakened, live virus (not the Ebola virus itself, but a modified livestock virus engineered to trigger an immune response against Ebola). It played a significant role in controlling outbreaks in the Democratic Republic of the Congo starting in 2018.
Treatments have also improved. Two monoclonal antibody therapies now exist that work by helping the immune system recognize and neutralize the virus more effectively. These treatments have meaningfully improved survival rates when given early in the course of illness. Supportive care, including aggressive fluid replacement and management of electrolyte imbalances, remains the backbone of treatment and makes a substantial difference in outcomes even without specific antiviral drugs.
Why Outbreaks Happen Where They Do
Ebola outbreaks cluster in Central and West Africa for a straightforward reason: that’s where the animal reservoirs live. Fruit bat species suspected of carrying the virus inhabit tropical forest regions across this belt of the continent. Human encroachment into forested areas, whether through logging, mining, or expanding agriculture, increases the chance of contact with infected animals.
Once a spillover event occurs, the trajectory of an outbreak depends heavily on local healthcare infrastructure. In settings with limited protective equipment, few isolation facilities, and traditional burial practices that involve washing the body of the deceased, the virus spreads rapidly. The 2014–2016 West Africa epidemic, the largest Ebola outbreak in history, killed over 11,000 people in Guinea, Liberia, and Sierra Leone. It demonstrated how quickly the virus can overwhelm fragile health systems and how difficult containment becomes once cases reach urban areas.