What Are Viruses, and Why Do They Make Us Sick?

A virus expert explains these tiny parasites as fears of coronavirus continue

We live with many viruses inside us already. Most viruses only cause problems when our immune system overreacts to their presence. Lightspring/Shutterstock

Editor’s Note: Some viruses cause the common cold, and some are crucial to human survival. As world health leaders try to determine how to respond to the new coronavirus, virus expert Marilyn J. Roossinck answers a few questions.

1. What Is a Virus?

Defining a virus has been a challenge, because every time we come up with a good definition, someone discovers a virus that breaks the rules. Viruses are entities that infect cellular life. They are very diverse. The simplest just have a couple of genes made of RNA or DNA wrapped up in a protein coat. Others have hundreds of genes, more than some bacteria.

All viruses are ultimately parasites. They require a host for replication. They cannot generate their own energy like cells can.

2. Why Does a Virus Make People Sick?

When a new human virus disease appears, it is most often because the virus has jumped from a different species into humans. The worst viruses are often the ones that have very recently jumped species.