Bill Gates says a pandemic flu could kill 30 million people — these are the deadly diseases that scare experts the most

REUTERS/David Moir

• A pandemic disease could be the greatest threat humanity faces right now.
• The WHO and CDC each keep lists of diseases that they consider urgent priorities for more research, but the next pandemic could be caused by a disease people have never seen before.
• For now, though, these are the six diseases that worry experts the most. 

For scientists that study disease, one unnerving fact is always present: somewhere on the planet, there's an organism or organisms — likely bacteria or a virus — that could kill tens of millions if it started to spread among people.

Pathogens we've never seen before lurk around the world, potentially living in bats, mosquitoes, or ticks, and waiting to jump to humans. Existing diseases mutate, becoming deadlier or more contagious. People are even growing more capable of modifying deadly diseases on their own.

Many experts think a global pandemic disease, whether naturally occurring or engineered, is the greatest threat humanity faces right now.

Bill Gates, who provides significant funding for disease research through the Bill and Melinda Gates Foundations, has repeatedly emphasized that we're not adequately prepared for the next pandemic illness. Recently, he warned that an outbreak of a disease like the flu virus that swept the world in 1918 could kill 30 million people within a six-month time span.

"The world needs to prepare for pandemics in the same serious way it prepares for war," Gates said at a recent discussion about epidemics.

The Centers for Disease Control and Prevention (CDC) keeps lists of the diseases that pose national security threats as possible biological weapon agents. The World Health Organization (WHO) also lists priority diseases that they think could become public health emergencies.

Here are some of the diseases that keep those experts awake at night.

The flu

The deadliest epidemic the world has ever seen took place 100 years ago, when the 1918 pandemic influenza virus blazed around the world. It infected  500 million people — a third of the world's population — and killed between 50 and 100 million people. 

In a normal year, the flu kills between 291,000 and 646,000 people around the world, according to the CDC. Experts think that if a new deadly flu like that 1918 pandemic virus were to emerge now, it would spread rapidly enough to kill almost 33 million people within six months.

Researchers have conducted experiments to see if it would be possible to make contagious flu strains deadlier or deadly flu strains more contagious. Those are viruses that no one ever wants to see in the wild.

Ebola, Marburg, Lassa, and other viral hemorrhagic fevers

Viral hemorrhagic fevers are spread by a variety of viruses and are mostly untreatable. They affect many organs and damage blood vessels, sometimes causing internal and external bleeding.

Infections often cause fever, weakness, soreness, and headache, often followed by vomiting, diarrhea, rashes, organ failure, and bleeding.

These diseases regularly emerge from the wild, as Ebola did again just recently. They can be extremely deadly — in past Ebola outbreaks, fatality rates ranged between 25% to 90%, with an average of about 50%.

Hemorrhagic fevers feature on both the CDC's list of biological agents that could pose a national security threat and on the WHO's list of diseases that can cause a public health emergency, and for good reason.

The 2014 Ebola outbreak in West Africa infected more than 28,600 people and killed more than 11,300. Such outbreaks can also cause public terror. Experts widely agree that the world needs to develop better ways to contain diseases like this. In the case of Ebola, researchers are testing an experimental vaccine to see if it can help stop the current outbreak.

If a strain of this type of disease were to mutate or be modified to become more contagious, that could cause worldwide devastation.

Coronaviruses like MERS and SARS

Coronaviruses are commonly found around the world, living in species like bats and camels. They take their name from crown-like spikes on the virus' surface.

For years, they were thought to be mostly innocuous. But in recent years, two coronaviruses that cause severe and potentially deadly respiratory symptoms in humans started to spread.

In 2002, SARS (severe acute respiratory syndrome) infected 8,000 people, killing approximately 10% of those patients. MERS (Middle Eastern respiratory syndrome) started infecting humans in 2012. While it is less contagious than SARS, it's more deadly, killing about 30% of those infected.

Most patients with these conditions first show signs of fever and many have gastrointestinal issues. These symptoms are often followed by shortness of breath and pneumonia.

This type of transition shows how quickly a new illness can emerge or transform to become a new public health threat — a process that regularly occurs. The WHO says there is an urgent need for research into ways to prevent the spread of these diseases.

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SEE ALSO: Bill Gates thinks a coming disease could kill 30 million people within 6 months — and says we should prepare for it as we do for war