As the coronavirus outbreak has spread to tens of thousands of people worldwide, it's become clear that one group isn't getting as sick as others: children.

No children under age nine have been reported dead in China and, so far, young kids make up less than one percent of confirmed cases there.

It's too soon to say exactly why children aren't getting as sick or dying of the virus that's killed more than 3,000 people worldwide, but experts suspect that their immune systems may be more familiar with other recent coronavirus strains.

That may confer some protection against COVID-19 to them.

But children's low rate of detected infections may also mean that they're insidious spreaders of the disease.

Children mainly fight off pathogens via an innate immune system that may be better at responding to a newly emerged infections, meaning they don't get as sick, but can pass on the infection to more vulnerable adults (file)

In the largest study conducted on coronavirus patients so far, researchers in China found that only 416 out of 44,672 people included in their report were under age 10 - accounting for just 0.9 percent of all infections.

One child between 10 and 19 out of 549 infected died.

Like most respiratory illnesses, death from coronavirus becomes increasingly probable with age.

Mortality rates stay below two percent for people under age 40. But between ages 60 and 79, those risks increase to 30 percent (although mortality falls back down to 20.3 percent for people over 80).

Underlying conditions and generally weaker constitutions and immune systems mean that respiratory viruses are harder on older populations.

But why children are relatively resistant to the infection remains unclear.

Two theories point to unique attributes of children's immune systems.

We actually have two types of immune systems: the innate and adaptive.

Humans and other vertebrates are born with innate immunity, a multifaceted system of defenses against pathogens.

There are now 103 people in the US with coronavirus, and six - all in Washington - have died

Innate immunity is 'non-specific,' meaning that these white blood cells attack anything foreign. They're blunter weapons than the components of the adaptive immune system, but are certainly better than no defense mechanisms.

Adaptive immunity is 'learned,' so to speak, as the body is exposed to more viruses and bacteria in the environment and develops weapons, called antibodies, specifically tailored to match various pathogens.

As we age, encounter bugs in the environment and develop more adaptive immunity, innate immunity fades.

So to older immune systems, the coronavirus is very new and very different, whereas for a young immune system, everything - or close to everything - is new, so the body is mounting a more similar defense response to what it's accustomed to doing any way.

And among the new pathogens that children are most innundated by are those in the coronavirus family, which are often the culprits of the common cold.

Children often pick up these viruses at school, which is why some Washington schools are shutting down amid its outbreak of the new infection and why CDC spokesperson Dr Nancy Messonnier suggested schools prepare for tele-classes.

Other coronaviruses currently circulating might be more similar to the one that causes COVID-19 than those to which adults had been exposed earlier in life.

What's more, children who pick up the new coronavirus and develop few or no symptoms are less likely to be tested, diagnosed and isolated.

And they're then more likely to spread it around to adults who are more vulnerable to the infection.

'It’s not that they’re not getting infection. They’re not getting disease. They’re not getting sick,' Dr Malik Peiris, a coronavirus expert at Hong Kong University told Stat News.

'If they are infected, there is no reason to believe that they will not transmit.

'There’s no evidence for any of this, but given the available data, that is quite a plausible scenario that one needs to think about.'