From animals to humans: Why HIV became a pandemic but Ebola didn't

Healthcare workers disinfect a stretcher after transporting a patient suffering from the Ebola virus disease at the Ebola Treatment Center in Bunia, in the east of the DRC, on June 23, 2026. [AFP]

When a Kenyan man began posting videos of himself eating from the same plate as wild birds, including scavengers, many social media users laughed.

Doctors did not.

To infectious disease experts, the images represented something far more significant than an internet spectacle. They highlighted one of the greatest public health threats of the 21st century the growing risk of zoonotic diseases, infections that spill over from animals into humans.

The discussion quickly gained momentum after KMPDU Deputy Secretary General Dennis Miskellah, publicly urged the Ministry of Health to intervene.

In a social media post directed at the Ministry, he wrote:

"Dear @MOH_Kenya, there is this character in FB who lives with wild birds including scavengers and eats with them from the same plate. While this looks funny, it won't be funny when he contracts and spreads zoonotic diseases. Kindly send public health officers to advice him accordingly coz we are not ready to pay for his folly with our lives!!"

His remarks sparked a heated debate.

Some Kenyans accused him of overreacting. Others defended his concerns, arguing that the world has learnt painful lessons about viruses that once circulated quietly among animals before infecting humans.

Importantly, there is no evidence that the individual has contracted or transmitted any disease. But the debate raises a scientific question that extends far beyond one man's unusual relationship with birds.

What happens after a virus jumps from animals into humans?

History shows that most zoonotic viruses disappear after infecting only a handful of people.

Some trigger outbreaks before dying out.

A few, such as Ebola, continue causing periodic epidemics.

Only one is known to have completely rewritten the rules.

Human Immunodeficiency Virus (HIV), which is believed to have crossed from non-human primates into humans in Central Africa early in the twentieth century, evolved into the world's largest sexually transmitted epidemic, infecting tens of millions of people and changing global public health forever.

Ebola followed a very different path.

Like HIV, Ebola is a zoonotic disease believed to have originated in wildlife, with fruit bats widely considered its natural reservoir. Human infection occurs after contact with infected animals before spreading through direct contact with blood and other body fluids of infected individuals.

Today, as the Democratic Republic of Congo battles another Ebola outbreak caused by the Bundibugyo strain, scientists continue asking the same question that has fascinated virologists for decades.

Why did HIV become a global sexually transmitted disease while Ebola did not?

One virus changed the rules

According to Dr Ahmed Kalebi, Consultant Pathologist, Founder and Chief Executive Officer of Dr Kalebi Labs and a PhD researcher in Public Health at the University of Cape Town, the answer lies in biology rather than chance.

"HIV is biologically adapted for efficient sexual transmission and lifelong persistence in the body. Infected individuals can remain asymptomatic for years while still transmitting the virus, allowing it to spread silently through populations before infection is recognised."

Those few years changed history.

Unlike diseases that rapidly make people seriously ill, HIV quietly established itself inside the human immune system.

People continued working.

They travelled.

They married.

They had children.

Many had no idea they were infected.

The virus was therefore able to move from one sexual partner to another long before medicine understood what it was dealing with.

Ebola behaves differently.

Its explosive nature is also what limits its long-term spread.

Dr Kalebi explains: "Ebola, in contrast, is primarily transmitted through direct contact with infected body fluids during acute illness. Most infected individuals become severely symptomatic within days, and many die if untreated, limiting opportunities for prolonged, undetected transmission. As a result, Ebola is far less efficient at sustaining widespread community transmission."

Unlike HIV, Ebola announces its arrival dramatically.

Patients rapidly develop fever, weakness, vomiting, diarrhoea and, in severe cases, bleeding.

Communities recognise something is wrong.

Health workers isolate patients. Contacts are traced. Transmission chains are interrupted. The virus burns intensely but often briefly.

Yet scientists have discovered something unexpected over the past decade.

Recovery does not always mean the virus has completely disappeared.

Following the devastating 2014-2016 West African Ebola epidemic, researchers began following survivors for months and even years after they had left hospital.

What they found surprised even seasoned virologists.

Although patients had recovered clinically, Ebola virus could still be detected in semen months later.

Some men remained positive for more than a year.

That discovery fundamentally changed how scientists understood Ebola recovery.

Dr Kalebi says although such cases remain uncommon, they cannot be ignored.

"Although Ebola virus can persist in semen for months after recovery and can occasionally be transmitted sexually, this is relatively uncommon and has not been the main driver of outbreaks. Consequently, unlike HIV, Ebola does not typically spread silently over wide geographical areas through asymptomatic individuals."

The finding nevertheless forced the World Health Organisation and affected countries to introduce long-term survivor monitoring programmes. Instead of ending follow-up when patients left treatment centres, health authorities began monitoring survivors, counselling couples and offering semen testing where available.

According to Dr Kalebi, this remains an important part of Ebola control.

"Sexual transmission accounts for only a small proportion of Ebola cases but can occasionally reignite outbreaks after apparent control."

He adds: "Survivors should undergo follow-up testing of semen where available and use condoms or abstain from sex until repeated tests confirm the virus is no longer detectable, while avoiding stigma because survivors themselves are not generally infectious in everyday contact."

Scientists describe certain organs in the human body as immune-privileged sites.

These include the testes, the eyes and parts of the central nervous system.

In these organs, immune responses are tightly controlled to prevent inflammation from damaging delicate tissues.

Unfortunately, viruses can sometimes exploit that protection.

Dr Kalebi says this explains why declaring an Ebola outbreak over requires much more than counting the number of patients discharged from hospital.

"It demonstrates that Ebola can evade complete immune clearance by hiding in immune-privileged sites such as the testes, eye and central nervous system. This means an outbreak may appear over clinically while a small risk of delayed transmission remains, highlighting the need for continued survivor monitoring and robust surveillance."