Viruses are considered non-living entities. They cannot reproduce by themselves. They need a host. Most are not looking to jump into humans. But why does a small fraction infect us?

Most new infectious diseases enter the human population the same way as COVID-19 did: as a zoonosis or a disease that is transmitted from animals to humans.

Mammals and birds are believed to host over 1.6 million undiscovered viruses. Don't panic. Although we live in a virus-filled world, we mostly remain disease-free.

But we are going to talk about one exception – a virus that successfully transits itself from one species to another. Scientists call it a spillover event. It's not as simple as just a jump and move, but involves evolution, eventually becoming a different virus. It's a long and complicated process.

For example, scientists found the closest virus to the novel coronavirus in nature is an existing strain detected in a bat a few years ago. Their genetic sequence is 96.2 percent identical. Despite the genetic difference of just 3.8 percent, scientists say it very likely represents more than 20 years of sequence evolution. At this time, it's difficult to know where the virus has been and what changes it has undergone during this period.

Scientists say the vast majority of animal-to-human spillover attempts likely result in a dead-end that never progresses past the first individual.

But given the large number of viruses that keep trying, there are always chances to slip through. Then they face the most challenging task – entering another species' cells. To do so, viruses need to grab onto the molecule outside of a cell, called a receptor, in a process similar to a key clicking into a lock and opening a door. Receptors vary from one species to another. So a virus that can use the human version of that receptor will become a zoonosis.

They didn't start out with that ability. But genetic flexibility allows it to happen, especially for coronaviruses that encode their genome as RNA instead of DNA. Without the proofreading function of DNA, RNA tends to have more errors, or mutations, when they are copying over their genetic code, creating more versions of themselves. Some of these may adapt to a new environment, like the human body.

In addition, some viruses can make up a new genetic code through a process a bit like sexual reproduction. Two viruses can swap segments of their genomes with each other and create a hybrid that differs from their "parents".

But no matter which method they adopt to mutate, the process needs time and space. Scientists believe an intermediate host was what offered the time and space for the new coronavirus to transfer from bat to human.

A research study examined the coronavirus-carrying bat, human and a potential intermediate host, the pangolin, checking the specific genome sequence that controls receptor binding.

That's the key factor in determining how infectious a virus is. It found that compared with its bat version, the virus in pangolins shows greater similarity to the one in humans. This suggests it may have used its time in the pangolin to alter itself so as to be a better fit for a human host.

Meanwhile, more research has revealed a new characteristic of viruses. They can hide in their hosts for long periods of time without being lethal.

Another theory is that this means the key mutation of a successful spillover could occur after the virus hops onto a human, instead of remaining in an animal.

If that's the case, the theory says the animal-to-human spillover could have occurred earlier, meaning a less severe version of COVID-19 has been "silently" spreading among humans long before the outbreak in Wuhan.