Since the first rumblings of an outbreak of respiratory illness in Wuhan, China, in December, researchers who study infectious diseases have had a handful of critical questions.

Three to four weeks may be a long time for anyone anxiously watching the news for fresh information on what is currently being called the 2019 novel coronavirus. But it’s not very much time for studying an outbreak.

“No one can look you in the eye” at this early stage and give you definitive answers to these questions, says Isaac Bogoch, an infectious disease specialist at the University of Toronto and Toronto General Hospital.

“We can speculate, we can use models, we can appreciate uncertainty in the data and prepare for various situations, but we do not have the answers.”

Here is what scientists are scrambling to find out.

How infectious is this illness?

Illnesses can spread more or less easily. A critical piece of information in tracking an emerging outbreak — but a difficult one to establish this early on — is the number of new cases created by a single sick person. Researchers call this the basic reproduction number, or R0 (“R nought”). Measles, an incredibly infectious disease, has an R0 of 12 to 18. In other words, each case results in between 12 to 18 new cases, which is why measles outbreaks are hard to contain (and why it’s so important that a large percentage of the population is vaccinated, blocking the disease’s spread). The seasonal flu, which is persistent but usually not overwhelming, has a mean R0 of 1.3.

The basic reproduction number is crucial for estimating how quickly an outbreak will grow, but can also change in response to external factors, like effective containment measures.

Multiple estimates for the R0 of the novel coronavirus have sprung up in the past few days, but most fall in the range of two to three, says David Fisman, a professor of epidemiology and infectious disease physician at the U of T’s Dalla Lana School of Public Health.

Two “is a scary reproduction number, because that means you’re doubling every six to 10 days,” says Fisman. On the other hand, “the cool thing about two is that it’s not that far from one. Which means if you nudge transmission down just a little bit, you actually get below that threshold of a reproduction number of one, which means it’s controllable.”

Experts all emphasize it’s very early days, and this number could change.

“It’s hard to be very confident in this, because you don’t have a lot of data,” says Bogoch.

How mild or severe are the illness’s symptoms?

Researchers have the same question as anxious members of the public: how sick will those who contract the virus get? A large number of mild cases can be both good and bad.

It’s good for obvious reasons — milder illnesses are preferable to severe ones — but also because it means the novel coronavirus may be less lethal than it appears to be. As of Tuesday, 106 people have died in China out of 4,515 confirmed cases. If it turns out there are significantly more people who are sick but who have slipped under the official radar because their symptoms are so mild, it means the percentage of lethal cases is smaller.

On the other hand, “If there are a significant number of people that may have the illness but may not be sick enough to seek medical care, they may be transmitting the infection in the community, and it might be harder to control,” says Bogoch. And an outbreak that is mostly mild but difficult to control can still result in a big death toll if huge numbers of people are infected.

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How many people are superspreaders?

After the SARS outbreak of 2003, studies showed that most people infected with the severe respiratory illness passed it on to no or few people. But a small number of people created large numbers of secondary cases — one documented “superspreader” was the source of 76 new cases, including in 12 health-care workers.

Again, the existence of superspreaders is both bad and good news.

“It’s bad because it obviously sucks if it happens in your jurisdiction,” says Fisman. “Perversely, it’s actually kind of good for control” — it’s easier to control a small number of significant outbreaks than it is to control many, many outbreaks.

Are people with no symptoms passing the virus on?

The phenomenon of people who are capable of spreading an infection but who show no symptoms of illness is known as “asymptomatic transmission.” The existence of asymptomatic transmission during an outbreak is a big deal — it makes the outbreak much harder to contain if people capable of passing on an infection are indistinguishable from healthy people.

Officials in China have reported an instance of asymptomatic transmission of the novel coronavirus — a young girl whose family was sickened and who didn’t appear to be sick herself. But researchers are highly cautious, saying much more evidence is needed.

“I would just like to see more data to support that statement,” says Bogoch. “Usually, people who truly have zero symptoms have little to no transmission of an infection, and people with more symptoms are typically more likely to transmit these infections.”

How is the disease spread?

It appears to be spread by coming into contact with respiratory secretions — in other words, by touching a surface that someone who is sick contaminated with droplets from their nose or mouth. That’s why public health officials in Canada are recommending frequent hand washing, rather than the surgical masks often pictured in news reports.

What animal did the virus jump from?

Most infectious diseases, including those that are new to humans, are “zoonotic”: they are the result of disease-carrying agents, or pathogens, that jump from an animal population to humans. SARS was eventually traced back to bats.

Researchers quickly disputed early reports that the novel coronavirus spilled over from snakes. The current best guess based on the genetics of the virus is that it also originated in bats, and may have jumped to another mammal before infecting humans. Knowing where the virus originated is not very important for containing the outbreak, but it is important for preventing the next one: markets like the one in Wuhan, that apparently sold wild animals, are perfect laboratories for creating outbreaks like this one, experts say.

“How many times can we learn this lesson? Apparently quite a number of times,” says Fisman.

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