It's a nightmare scenario: An infectious disease is spreading around the world and threatening to topple civilization as we know it. But what kind of disease could do this?

A new report aims to address that question, in hopes of preventing or better preparing for such a scenario. The researchers found that although pathogens like Ebola and Zika make headlines, they are unlikely to cause a global pandemic disaster. Instead, viruses that are spread through the air — including those related to the common cold virus — pose a bigger threat, even though some of these viruses don't receive much attention. (Ebola and Zika are spread through other means, including contact with bodily fluids and, for Zika, mosquitoes.)

"We need to get serious about respiratory viruses," said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security in Baltimore, who led the report. "[There's] a lot of focus on diseases that aren't going to be able to change civilization in a way that something that's spread through the respiratory route would be." [10 Deadly Diseases That Hopped Across Species]

Historically, authorities have prepared for pandemics by focusing on a list of "usual suspects" — diseases that have caused outbreaks in the past, such as the flu and SARS (severe acute respiratory syndrome), or those that could be used as biological weapons. But this approach doesn't account for pathogens that aren't currently known or haven't historically caused outbreaks, the researchers said.

So, for the new report, the researchers essentially started from scratch, without any preconceived notions of what the most likely culprit of such pandemics would be. They reviewed the literature on the pathogenic potential of microorganisms — in other words, the likelihood that the germs could rapidly spread — and related topics, and they interviewed more than 120 experts in the field.

The report found that, in addition to being airborne, a pathogen with the potential to cause a global pandemic disaster would likely have the following traits:

It would be contagious during the "incubation period," before people show any symptoms, or when people have only mild symptoms.

It would be a microbe that most people are not immune to, so there would be a large population of susceptible human hosts.

It wouldn't have an existing treatment or prevention method.

It would have a "low but significant" fatality rate.

Although the final trait may sound surprising, Adalja noted that a pathogen doesn't have to have a high fatality rate, or kill the majority of people infected, to cause majority societal disruptions. "It just has to make a lot of people sick," he told Live Science. (A pathogen with a high fatality rate could kill too many people too quickly, and therefore run out of "hosts" to spread further, the report noted.)

Indeed, the infamous "Spanish" influenza pandemic of 1918 had a fatality rate of just 2.5 percent, but because it infected hundreds of millions of people, it caused an estimated 50 million deaths, according to the Centers for Disease Control and Prevention. In addition, according to something called the "host density threshold theorem" a virus that kills too many people will "run out of susceptible hosts and be extinguished," the report said.

The report also found that a group of viruses known as RNA viruses have the most potential to cause a global pandemic disaster, in part because these viruses mutate more easily than other types do. This group includes well-known viruses such as the flu (influenza) and SARS, but also common cold viruses, such as enteroviruses and rhinoviruses, as well as respiratory syncytial virus.

While the flu has received a lot of attention for its ability to cause pandemics, many other viruses in this group have not. There's "a whole host of viral families that get very little attention when it comes to pandemic preparedness," Adalja said.

The report called for improved surveillance of RNA viruses, similar to what's done for influenza.

In addition, the report recommended an increased emphasis on developing antiviral drugs against RNA respiratory viruses, as well as vaccines, including a universal flu vaccine.

Another important strategy to get in front of pandemic pathogens will be greater testing of patients to pinpoint the exact infectious cause of their symptoms, the report said. Often, when patients come to the hospital with certain symptoms, such as a cough or difficulty breathing, they are diagnosed as having "pneumonia" or a "viral syndrome" without any testing done to confirm exactly what pathogen caused the illnesses. But the report said that such testing should be routine, because "it is unclear where the next pandemic pathogen will appear."

Adalja pointed out that the initial cases of a global pandemic may be relatively mild. "[It's] not always going to be somebody dying a horrible death … it could be a very minimal case," symptoms-wise, Adalja said.

For example, the first case of the 2009 H1N1 pandemic was identified in a child who had flu-like symptoms but did not have a severe illness. "That could happen again," Adalja said.

Original article on Live Science.