A coronavirus pandemic is sweeping the world, but what exactly is a virus? Is it alive?

You already know a new virus has brought normal life to a halt in the United States. But you may not remember what a virus even is.

They're invisible and can make us sick - but how? And why is it so hard for scientists to stop a new virus?

There are many different types of viruses, including ones that affect animals, plants and other organisms, so it's often a challenge to nail down an answer to even simple questions.

The answers below won't explain how every virus behaves, but in a time when it's easy to feel powerless and confused, they will help you understand why fighting the coronavirus pandemic is such a challenge.

What does the coronavirus do to your body? Everything to know about the infection process

What is a virus?

A virus is a microscopic piece of genetic material surrounded by a coat made of proteins. It enters healthy cells and hijacks them, creating copies of itself.

When viruses begin replicating inside a living organism, they can cause an infectious disease. In the case of the coronavirus pandemic, the virus is SARS-CoV-2, and the disease is called COVID-19.

Are viruses alive?

It's complicated.

The National Human Genome Research Institute describes viruses as existing "near the boundary between the living and the nonliving."

That's because viruses can't function without interacting with a living cell. On their own, they're also essentially inert - unable to move - as a 2017 study notes.

"By themselves, they can't do anything. They need a host cell to replicate," virologist Paulo Verardi told USA TODAY. Verardi works on vaccine development and is a University of Connecticut professor.

He suggested thinking of them like a parasite: an organism that survives by harming another species.

But definitively answering whether a virus is alive may be more of a philosophy question than one strictly for science, Verardi said.

How do you kill a virus?

If it's outside your body, soap can kill a virus. Once the virus begins replicating inside your body, it's much harder.

Most viruses, especially respiratory viruses, are easily "disassembled" by soap when they are outside your body, Verardi said.

As long as you scrub your hands vigorously and rinse well with water, the soap essentially kills the virus.

Once the virus begins to take hold in your body, it's up to your immune system to clear it out.

There's two main ways this is done, Verardi said. First, the body can attempt to attack the virus directly, stopping it from hijacking cells and spreading rapidly.

And second, the body can try to spot its own cells that are infected with the virus and kill those cells. That's obviously not ideal and can cause damage to your body - but it's often necessary to stop the spread of the virus.

How and why does a virus make us sick?

The specifics of this will vary based on the virus. But broadly, Verardi says, you should think of the interaction between the virus and your body as a war.

As a virus replicates in your body, two damaging processes are at play.

The first: The virus is infecting cells and using them to replicate itself - this process often kills the infected body cells, causing damage to the body.

At the same time, the immune system is trying to clear the virus from the body. If too many cells are infected, the immune system's response - targeting infected cells - can also be harmful.

This battle can cause all sorts of problems in our body, depending on the virus and its location: inflammation, fever, mucus and more can occur.

In many cases, our bodies win the battle - viruses like the flu or the common cold are usually fairly easy for a healthy person to beat.

But some viruses can be much harder to fight, especially for people with compromised immune systems.

Can vaccines or medicines help fight a viral disease?

Yes, but typically only when they target a virus specifically.

It's something like the relationship between a key and a lock: You can't use any key to get the desired result.

What makes things worse: As viruses replicate rapidly, some of them mutate. When that happens, vaccines and treatments must account for a virus that doesn't stay the same.

That's the case for the flu and why there is a new flu shot every year, Verardi said.

Drugs, specifically antiviral medications, can help fight viruses once a person is infected. But they work best before a virus hijacks too many of the body's cells. Again, the same mutation complication often applies.