What it is

Bundibugyo ebolavirus is one of six known species in the genus Orthoebolavirus, family Filoviridae. It was first identified in 2007 during an outbreak in Bundibugyo District, Uganda — the district it's named for.

It is a zoonotic disease: the virus normally circulates in wildlife and occasionally spills over into humans, after which it can spread from person to person.

Transmission

Initial infection follows contact with the blood or secretions of infected wildlife — bats or non-human primates. Human-to-human spread then occurs through direct contact with blood, secretions, organs, or other bodily fluids, and is amplified in healthcare and funeral settings.

Symptoms

Sudden fever, fatigue, muscle pain, headache, and sore throat, typically followed by vomiting, diarrhoea, rash, and signs of impaired kidney and liver function. Some patients develop internal and external bleeding.

Why this species is different

What it is

Marburg virus and the closely related Ravn virus cause Marburg virus disease. It is Ebola's closest relative — the two are the only genera in the Filoviridae family known to cause severe hemorrhagic fever in people.

The virus was first identified in 1967, after simultaneous outbreaks among lab workers in Marburg and Frankfurt, Germany, and in Belgrade, Serbia — all traced to African green monkeys imported from Uganda for vaccine research.

Transmission

People are usually infected after prolonged exposure to bat colonies in caves or mines. Once a person is infected, the virus spreads through direct contact with blood, secretions, organs, or other bodily fluids — including during caregiving and burial practices.

Symptoms

Abrupt high fever with severe headache and malaise, often with severe watery diarrhoea, abdominal pain, nausea and vomiting. Many fatal cases develop bleeding, often from multiple sites, five to seven days in.

How it compares to Ebola

What it is

Andes virus is a hantavirus native to southern South America. In humans it causes Hantavirus Pulmonary Syndrome (HPS): a severe, often fatal lung disease. Hantaviruses have co-evolved with rodent hosts for millions of years, causing no illness in the animal itself.

It was identified as the cause of a 1996 outbreak in Argentina — the first time any hantavirus had been shown to spread between people, a property no other hantavirus on Earth is confirmed to share.

Transmission

Most infections follow inhaling aerosolized particles from rodent urine, droppings, or saliva, often while disturbing dust in enclosed rural spaces. Andes virus can also spread person to person through close, prolonged contact — the mechanism behind this shipboard cluster.

Symptoms

An early phase of fever, headache, muscle pain, and fatigue, which can progress within days to sudden, severe respiratory failure as fluid fills the lungs.

Why this one is different

What it is

Yellow fever virus is a flavivirus, related to dengue and Zika. It takes its name from the jaundice that colours the skin and eyes in severe cases. The virus was isolated in 1927 in Nigeria; the resulting 17D vaccine, developed in the 1930s, remains in use today and won its creator, Max Theiler, a Nobel Prize.

Transmission

Spread entirely by mosquito bite, never person to person. A sylvatic (jungle) cycle circulates between wild primates and forest mosquitoes; an urban cycle passes directly between people via Aedes aegypti in cities. Nearly every outbreak begins when the two cycles meet.

Symptoms

Sudden fever, chills, headache, and back pain. Most recover after this initial phase. A minority relapse into the severe, "toxic" phase — jaundice, bleeding, and organ failure — which carries the higher fatality rate above.

The outlier on this list

What it is

Dengue virus has four distinct serotypes, and infection with one gives only partial, temporary protection against the others — a second infection with a different serotype is often more severe than the first.

It's the most common mosquito-borne viral disease in the world, with an estimated 100–400 million infections a year across the tropics and subtropics.

Transmission

Spread entirely by the bite of infected Aedes aegypti or Aedes albopictus mosquitoes, which breed in small pockets of standing water even in dense cities. There is no person-to-person spread.

Symptoms

High fever, severe headache, pain behind the eyes, joint and muscle pain severe enough to earn the nickname "breakbone fever," and a rash. A small share progress to severe dengue, with bleeding, plasma leakage, and shock.

Why the vaccines are complicated

What it is

Vibrio cholerae is a waterborne bacterium, not a virus. It produces a toxin that causes the body to expel enormous volumes of fluid — the defining danger of cholera is rapid, severe dehydration, not the infection itself.

Outbreaks track closely with breakdowns in clean water and sanitation infrastructure — after conflict, displacement, flooding, or the collapse of water systems.

Transmission

People are infected by swallowing water or food contaminated with the bacterium, typically traced back to sewage-contaminated drinking water. It is not spread through casual person-to-person contact.

Symptoms

Sudden, profuse watery diarrhoea and vomiting, leading to dehydration that can become life-threatening within hours without treatment.

The most treatable disease on this list

What it is

Measles virus is among the most contagious pathogens known to science — a single infected person can pass it to 12 to 18 others in a fully susceptible population, several times the spread rate of most other respiratory viruses.

Because it spreads so efficiently, outbreaks are an early warning sign: measles resurges the moment vaccination coverage in a community drops, even slightly, below the roughly 95% needed for herd immunity.

Transmission

Airborne — through coughing and sneezing. The virus can linger in a room for up to two hours after an infected person leaves it.

Symptoms

High fever, cough, runny nose, and red, watery eyes, followed by a characteristic rash that spreads from the face down the body. Complications include pneumonia and, rarely, brain swelling.

A near-solved problem that keeps returning

What it is

Monkeypox virus is a relative of smallpox, first identified in captive research monkeys in 1958 despite its animal reservoir never having been pinned down. Two genetically distinct clades exist: Clade I (Central Africa), historically far more severe, and Clade II (West Africa), which caused the 2022 global outbreak.

Transmission

Zoonotic spillover — likely from rodents — starts most Central African outbreaks. From there it spreads person to person through close physical contact, respiratory droplets, and contact with lesions or contaminated materials.

Symptoms

Fever, headache, muscle aches, and swollen lymph nodes, followed by a distinctive rash that progresses through raised bumps to fluid-filled blisters before scabbing over.

One name, two very different diseases

What it is

Lassa virus is endemic across much of West Africa, where the rodent that carries it is extremely common around human settlements. Most infections — about 80% — are mild or entirely without symptoms, which lets the virus circulate quietly for long stretches between visible outbreaks.

Transmission

Mainly through contact with food or household items contaminated by rodent urine or droppings, or by eating the rodents themselves. Person-to-person spread can follow, through contact with an infected person's blood or bodily fluids.

Symptoms

Gradual fever, weakness, and malaise in most cases. A minority progress to severe disease, with facial swelling, bleeding, and hearing loss — Lassa fever is a leading cause of acquired deafness in West Africa, even among survivors.

A quiet, constant risk

What it is

SARS-CoV-2 is a coronavirus first identified in late 2019. Mainstream virology points to a zoonotic spillover, most likely from a bat-origin coronavirus, though the precise animal intermediary and the exact circumstances of the first human infections remain scientifically debated.

It's now considered endemic in most of the world — circulating continuously at lower, more predictable levels rather than the sharp emergency waves of 2020–22.

Transmission

Primarily airborne, through respiratory droplets and smaller aerosols released when an infected person breathes, talks, coughs, or sneezes.

Symptoms

Fever, cough, fatigue, and loss of taste or smell were hallmark early symptoms; presentation has shifted with each new variant. Most cases are now mild, especially in the vaccinated.

The fastest countermeasure ever built