How does the new coronavirus disease, Covid-19, spread? That’s just one of many basic, tricky questions about this latest pandemic threat.

The virus that causes Covid-19 — known as SARS-CoV-2 — has already infected many people around the world in just a few months. And the best explanation for this rapid spread is that the virus is being passed through droplets from coughing or sneezing. When these virus-laden droplets from an infected person reach the nose, eyes, or mouth of another, they can transmit the disease.

But are there other ways SARS-CoV-2 moves between people? And what do they tell us about why this disease seems to be even more contagious than SARS and MERS? The latest science on the virus offers possible answers to these questions — and why Covid-19 might be particularly difficult to stop. Here’s what we know so far.

People have lots of virus in their bodies early on in illness — or even when they have no symptoms

Respiratory illnesses generally fall into two categories: upper respiratory — infections in the nose, pharynx, or larynx, like the common cold and seasonal influenza; and lower respiratory illnesses, like pneumonia, which infect the lungs.

The original SARS virus that spread around the world in 2003 was thought to be a lower respiratory infection: It replicated in the cells deep within the lungs and caused the pneumonia. People also seemed to only spread the virus days into their illness, when it was already clear they were sick. This made SARS more difficult to pass on to others and the job of containing it relatively easy.

The new virus that causes Covid-19 disease appears to be a different beast: While it also can eventually lead to pneumonia, the virus does a great job of replicating in the upper respiratory tract, even when people don’t have any symptoms or just begin to feel sick.

Check out this New England Journal of Medicine paper. Chinese researchers monitored how much virus could be found in the upper respiratory tracts — noses and throats — of 18 patients in Guangdong, China. One of the 18 never had any symptoms.

The big finding? The way people shed this virus, potentially exposing others, looked a lot more like the flu than the first SARS, which might help explain why Covid-19 appears to be more infectious. You can see why in this chart from the study, focused on the patients who experienced symptoms: Just as they were starting to feel ill, they had the highest concentrations of virus in their noses:

What’s more, the one person who was asymptomatic carried a similar amount of virus as the symptomatic patients, “which suggests the transmission potential of asymptomatic or minimally symptomatic patients,” the researchers wrote.

In a separate New England Journal of Medicine paper, researchers in Germany were also able to isolate the virus from patients’ upper respiratory tract even before they showed any symptoms or were very mildly symptomatic — more evidence of the potential for spread of the virus from the nose and throat when people barely know they’re sick.

A yet-to-be published paper, also from German researchers, looked at nine of the country’s earliest confirmed Covid-19 patients. They too found evidence of the virus replicating in the upper respiratory tract — but that wasn’t the most interesting takeaway. The researchers discovered people generally aren’t infectious after about 10 days, and people with mild symptoms stop shedding the virus after about a week. But, unlike much of the data from China to date — which suggests most people present with fever and a dry cough — symptoms for many people in this group resembled a cold. So only two of the nine had a fever and seven had a cough, but just as common were symptoms like stuffy nose, runny nose, and sneezing.

What this means for stopping Covid-19

What does this imply about the contagiousness of Covid-19 and stopping the outbreak? “For a virus pretty closely related to SARS, it shows very effective person-to-person transmission, something nobody really expected,” Stephen Morse, a professor of epidemiology at Columbia University Mailman School of Public Health, told Vox.

Researchers currently believe one infected person generally infects two to more than three others, which would make the new coronavirus more contagious than seasonal flu, SARS and MERS.

Second, it means stopping the outbreak might be more difficult, since people start to become infectious early on in their disease or may even spread the virus when they’re asymptomatic or after symptoms go away.

But to confirm these findings, we’ll need more science, said Jennifer Nuzzo, an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security. “We still don’t know to what extent people without symptoms can infect,” she pointed out.

It’s also possible that transmission early in the illness or from asymptomatic people won’t end up being important contributors to the outbreak, said Marion Koopmans, who studies emerging infectious diseases and heads the department of virology at the Erasmus Medical Center in Rotterdam, Netherlands. In most parts of the world where travelers with Covid-19 turned up, she added, the spread of the disease has been contained by only testing people with symptoms. But, she added, “both of these statements can coexist: Asymptomatic shedders could spread the virus, but it probably is not the main driver of this epidemic.”

The virus might spread through feces

Another way viruses can spread is through poop. Think of the norovirus, the extremely contagious bug that can be passed along by ingesting the stool of an infected person, often through food or touching a contaminated surface. This is known as the “fecal-oral” route of disease transmission.

Now there’s some suggestion in the emerging literature that Covid-19 could be passed through exposure to virus-laden feces, too.

In this new paper from the Chinese Center for Disease Control and Prevention, researchers managed to isolate live virus from stool samples of Covid-19 patients. And they’re not the first to find the virus in stool.

As with norovirus, this means the disease could be passed around when there’s less than optimal hygiene. “If true, it would not be surprising,” Morse said. “A number of other coronavirus are excreted from the intestines, and infectious virus can be found in stool.”

That’s why the China CDC recommended taking measures to stop the spread of the virus this way, including:

maintaining environmental health and personal hygiene; drinking boiled water, avoiding raw food consumption, and implementing separate meal systems in epidemic areas; frequently washing hands and disinfecting of surfaces of objects in households, toilets, public places, and transportation vehicles; and disinfecting the excreta and environment of patients in medical facilities to prevent water and food contamination from patients’ stool samples.

But just because the virus is found in stool doesn’t mean that’s how it’s transmitting. And, again, more research is needed to figure out how important the fecal-oral route is in the spread of this disease.

Airborne transmission: One more thing to watch out for

Poop was also implicated in the first SARS outbreak, when a large housing estate in Hong Kong called Amoy Gardens became ground zero of a public health nightmare. More than 300 people were infected with the disease through yet another viral transmission route: airborne transmission of virus-ridden feces aerosols.

Airborne spread happens when the residue from evaporated, infected droplets gets suspended in the air and indirectly infects those who breathe it in. It’s different from droplet transmission, since droplets are too large to float through the air and need to get sprayed directly on someone’s eye, nose, or mouth in order to infect them.

In the case of Amoy Gardens, researchers learned SARS was capable of going airborne, spreading through the building’s faulty plumbing and ventilation systems to the people who lived on the estate. “The infections [were] officially attributed to faulty toilet traps which were thought to have aerosolized patients’ virus when the toilet was flushed, allowing dispersal of the virus to other residents,” Morse explained. “This has been demonstrated with SARS and MERS and others, and therefore is plausible, although we currently lack good evidence.”

So researchers and doctors are looking into whether the new SARS virus spreads this way and taking precautions in case it can. Vito Iacoviello, chief of the division of infectious diseases at Mount Auburn Hospital in Cambridge, Massachusetts, and an editor at Dynamed, noted that the US Centers for Disease Control and Prevention is recommending people admitted to hospitals with Covid-19 be put in an airborne isolation room. “That’s the precaution we use for TB, measles, and chickenpox,” he said, and it suggests health officials are preparing for the possibility that this virus is capable of airborne spread.

For now, there’s no good evidence of Covid-19’s airborne transmission. It’s just another thing to watch out for as our understanding of this virus, and how it moves through populations, evolves.