"Novel coronavirus" is the proper term for this brand-new virus wreaking havoc on our unprepared world.

But you can also call this nasty villain by its scientific name: severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2 for short.

Covid-19 seems to strike the elderly and immunocompromised the hardest, along with any of us with underlying health conditions such as diabetes, heart and lung disease. But the young shouldn't take anything for granted -- there have been numerous deaths among people aged 20 to 50, as well as a very few among children.

Covid-19 can also present with mild symptoms very similar to a typical cold or flu -- or no symptoms at all, which makes controlling the spread of the virus causing Covid-19 very difficult.

What is a coronavirus?

There they set up shop, producing millions of copies of themselves and causing those cells to rupture. Like the famous scene from the movie "Alien," the viral offspring shoot out into the bloodstream, with the goal of invading more and more cells.

As they multiply, humans began to spit them out into the universe with each exhalation, making us contagious days before we begin to cough, sneeze or have diarrhea -- all symptoms the virus creates to ensure it can leap from human to human, thus ensuring its survival.

This "virus zombie invasion" comes in all sort of shapes, sizes and genetic strategies. All coronaviruses are covered with pointy spires of protein, giving them the appearance of having a crown or "corona" -- hence the name. Coronaviruses use these spikes to latch onto and pierce our cells.

Coronaviruses are part of the RNA brigade of viruses, which are much less stable than their DNA-based comrades. Why is that important? Because instability leads to mistakes in copying genetic code.

That leads to mutations -- thousands, millions, billions of mutations. Sooner or later, one mutation hits pay dirt and allows the virus to cross the great divide between different species. A few million/billion/trillion more mistakes creates another mutation that allows that virus to spread easily. Now the virus is both in its new host and it is contagious.

It's that type of mutation which gives humanity viruses like SARS-CoV-2.

Where did the novel coronavirus come from?

Some of those coronviruses can cross species, such as between pigs, cats and dogs, but for the most part coronaviruses stay loyal to their original hosts. Until, of course, they become that lucky mutation.

"Usually viruses from one animal really don't effectively transmit to other animal species or even to people," said Dr. John Williams, chief of the division of pediatric infectious diseases at the University of Pittsburgh Medical Center Children's Hospital of Pittsburgh.

"So usually if a virus goes from an animal to a human, it's sort of dead end. That person gets sick but it doesn't spread further," said Williams, who has studied coronaviruses for decades.

"MERS is extremely deadly, about 30% of people who are infected with MERS will die," Williams said. "So the virus got over one of the barriers -- it's able to infect humans, grow in them and cause disease -- but thankfully it really doesn't spread well person to person, other than very, very close contacts."

SARS has been more difficult to pin down.

"SARS caused death in about 10% of people that became infected and it did spread person to person but not super effectively," Williams said. "There weren't many people walking around without symptoms or with mild symptoms, who could be spreading it.

"This new virus, SARS-CoV-2, has overcome more barriers," Williams added. "It spreads easily person to person and a lot of people can have either mild disease or they might not even have symptoms, yet they can have the virus and spread it."

At this time, scientists don't know where the novel coronavirus began.

"These things are more difficult than [identifying] dinosaurs, because there's no fossil record of a virus," Williams said. "For example, the main virus I study, human metapneumovirus, is clearly a virus that has circulated in humans for decades if not a few centuries.

"However, when you look at the genetics of the virus, its closest genetic relative is a bird virus," he added. "So, did that virus jump to humans way back and become established? That's what we think. But it isn't impossible that a human virus jumped to birds and became established there."

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What is coronavirus and Covid-19? An explainer - CNN