They're incredibly abundant.

There are more viruses in a single drop of seawater than there are people living in New

York City.

And there are more viruses on Earth than there are stars in the universe.

They've had a lot of time to become so plentiful.

Viruses were here long before we were.

Some scientists believe they're older than any life on Earth.

They are also incredibly diverse.

Viruses can invade all types of life, from animals to plants, bacteria to fungi.

But what are they, exactly?

Viruses aren't technically alive--though that's still a matter of scientific debate;

they're merely packets of genetic information.

Some carry that code in single strands of RNA, others in double strands or in DNA.

But they all use those instructions as a blueprint for invading healthy cells, or “hosts.”

They enter and take over a cell to replicate: either destroying the host cell's genetic

code or splicing themselves onto it.

After replicating, viruses escape to hijack other cells, often destroying their host cells

in a process called “lysis.”

This isn't always a bad thing.

By breaking and restructuring other cells—like ones inside phytoplankton in the ocean—viruses

play an important role of increasing the efficiency of the Ocean's “biological pump”: the

process of sequestering carbon dioxide from the atmosphere to the ocean floor.

An ancient virus also gave rise to the human placenta, which is why our babies aren't

born from eggs.

But those were happy side effects.

Viruses only care about one thing: making more viruses.

And that often causes big problems.

Starting in the late 1800s, scientists discovered many diseases spread by unknown, unseen pathogens.These

mysterious infections were destroying tobacco plants and infecting livestock across Europe

and the United States.

In 1902, American physician Walter Reed discovered that a similar agent had been the cause of

the Yellow Fever epidemic.

Researchers began calling these pathogens “viruses,” Latin for poison.

At the time, Reed and his peers could isolate viruses with cutting-edge filters because

they happened to be much smaller than bacteria.

But they couldn't actually see them with a regular microscope.

The tobacco mosaic virus would be the first to come into actual view with the invention

of the electron microscope in the 1930s.

Once we could see viruses, we started to realize that they were everywhere, including inside

our own bodies.

Viruses have replicated inside humans throughout our evolution.

That's why about eight percent of the human genome is derived from viruses—remnants

of infections in our ancient ancestors.

These viruses are generally inactive, or “latent,” though some can be reactivated by autoimmune

diseases, neurodegenerative conditions, chronic inflammation, and cancer.

In addition to going latent, sometimes for decades, viruses can also mutate.

This can happen as a result of physical damage, chemical changes within the virus's genetic

material, or errors during the replication process.

Mutation is the reason that the novel coronavirus, SARS-CoV-2, was able to jump from infecting

bats to infecting humans—something known as “host-switching.”

Coronaviruses in particular have a unique way of mutating: they readily swap bits of

RNA with other coronaviruses in a process called “recombination.”

That makes it easy for the virus to adapt to new species.

The coronavirus pandemic has brought the adaptability of viruses into full view.

What remains somewhat hidden is how many more deadly viruses like it might be waiting to

jump from animals into humans.

Some estimates give reason for concern.

A 2018 study found that of 111 viral families identified worldwide, there are some 631,000

to 827,000 yet-unknown viruses that “likely have the capacity to infect people.”

It's unclear how many of them could one day pose a threat to humans.

But what is clear is that we still have so much to learn about the multitudes of viruses

that share the world with us.