Ebola is caused by infection with an ebolavirus, a group of closely related viruses in the filovirus family. Four distinct species cause disease in humans, with the Zaire species being the most common and deadliest, carrying an average case fatality rate of 75%. The virus originates in animals, most likely fruit bats, and spreads to humans through direct contact with infected bodily fluids.
The Virus Behind the Disease
Not all Ebola outbreaks are caused by the same virus. Four separate ebolavirus species are known to sicken humans, and they vary significantly in how dangerous they are. The Zaire ebolavirus is responsible for most major outbreaks, including the devastating 2014 West African epidemic, and has the highest fatality rate at roughly 75%. Sudan ebolavirus kills about 53% of those infected. Bundibugyo ebolavirus, first identified in Uganda in 2007, is the least severe at around 34%. A fourth species, Taï Forest virus, has caused only a single known human case.
These differences matter because the species circulating in a given outbreak shapes how aggressively public health teams respond and what treatments they deploy. The two antibody-based drugs currently approved to treat Ebola, for instance, specifically target the Zaire species.
Where the Virus Comes From
Ebola is a zoonotic disease, meaning it jumps from animals to people. The leading suspects for the virus’s natural reservoir are fruit bats found across Central and West Africa. In 2003, researchers investigating an outbreak in Gabon detected viral genetic material in three species of fruit bat: the hammer-headed bat, Franquet’s epauletted fruit bat, and the little collared fruit bat. Since then, Ebola-specific antibodies have been confirmed in several additional bat species across multiple countries, though the percentage of bats carrying these antibodies tends to be low and varies by location.
Despite more than 1,500 individual bats analyzed across the three primary suspect species, researchers have not yet isolated live virus directly from a bat. This is the gap that keeps the reservoir question officially open. Still, the weight of evidence, including antibody detection, viral RNA fragments, and the ecological overlap between bat habitats and outbreak zones, points strongly toward fruit bats as the source. Humans likely become infected through contact with bat saliva, droppings, or partially eaten fruit, or by handling intermediate hosts like primates and forest antelope that were themselves infected by bats.
How Ebola Spreads Between People
Once the virus enters the human population, it spreads through direct contact with the bodily fluids of someone who is sick or has died from the disease. The relevant fluids include blood, vomit, feces, urine, saliva, sweat, breast milk, and amniotic fluid. Ebola does not spread through the air like influenza or measles.
Infection requires the virus to reach broken skin or the mucous membranes of your eyes, nose, or mouth. This is why healthcare workers and family caregivers face the highest risk, especially when protective equipment is scarce. Objects contaminated with infected fluids, such as bedding, clothing, needles, and medical equipment, can also transmit the virus. The virus survives longer on surfaces in cooler, more humid conditions (like hospital environments) than in the hot, dry conditions typical of many African settings, and longer in liquid blood than in dried blood.
One transmission route that persists after recovery is sexual contact. The virus can remain in semen for months after a person has otherwise recovered, making it possible to pass the infection through oral, vaginal, or anal sex well after other symptoms have resolved.
How the Virus Infects Your Cells
Ebola uses an unusual strategy to get inside human cells. The virus is studded with a surface protein that attaches to cells and triggers the cell to pull the virus inward, essentially swallowing it into a small internal compartment. Once inside, enzymes in the cell strip down that surface protein, exposing a hidden region that latches onto a cholesterol-transport protein embedded in the compartment’s wall. This binding event is the key that unlocks fusion, allowing the virus to release its genetic material into the cell and begin replicating.
This entry mechanism is relevant beyond the biology classroom. Both FDA-approved Ebola treatments work by blocking this exact process. One drug uses three antibodies that simultaneously bind to different spots on the virus’s surface protein, preventing it from attaching to cells. The other blocks the specific interaction between the virus and that cholesterol-transport protein inside the cell. In clinical trials, both drugs significantly reduced mortality compared to earlier experimental treatments, with one lowering the 28-day death rate from about 49% to 35%.
Why Ebola Is So Deadly
The virus is lethal in large part because it actively disables your immune system while it replicates. Two viral proteins play central roles. One prevents your cells from detecting the virus’s genetic material and blocks the chemical signals (interferons) that would normally sound the alarm to neighboring cells. The other protein stops a key immune messenger from entering the cell nucleus, where it would activate defensive genes. Together, these proteins create a window of unchecked viral replication during the critical early days of infection.
With the innate immune response suppressed, the virus rapidly infects immune cells, liver cells, and the cells lining blood vessels. This leads to the hallmark features of severe disease: massive inflammation, organ damage, disrupted blood clotting, and in many cases, hemorrhaging. By the time the adaptive immune system mounts a meaningful response, the damage can be overwhelming.
Incubation Period and Early Symptoms
After exposure, symptoms can appear anywhere from 2 to 21 days later, with most people developing their first signs between 8 and 10 days. A person is not contagious until symptoms begin, which is one reason Ebola outbreaks, while devastating, tend to be more containable than airborne diseases.
Early symptoms are nonspecific: fever, fatigue, muscle pain, and headache. This makes the first few days difficult to distinguish from malaria, typhoid, or other common tropical infections. As the disease progresses, it causes severe vomiting, diarrhea, and in some cases internal and external bleeding. Death, when it occurs, typically results from multi-organ failure and shock due to massive fluid loss and immune system collapse.
Vaccination and Prevention
A vaccine targeting the Zaire ebolavirus species has been in use since 2019. During the 2018 to 2020 outbreak in the Democratic Republic of the Congo, the vaccine demonstrated 84% effectiveness against Ebola virus disease when measured 10 or more days after a single dose. It is now a standard tool deployed in ring vaccination strategies, where contacts of confirmed cases and their contacts are vaccinated to create a buffer zone around new infections.
The vaccine does not cover the Sudan or Bundibugyo species, which means outbreaks caused by those viruses, like the 2022 Sudan ebolavirus outbreak in Uganda, still lack a proven vaccine. Research into the duration of protection from the existing vaccine is ongoing, particularly for vulnerable groups like children and pregnant women.