What Are Viruses?

COVID-19 is a virus that causes respiratory symptoms in some people and severe pneumonia — even death — in others. But what exactly is a virus? What makes it different from a bacteria, and why are there no antibiotics for it?

Viruses are not quite “alive”

The word virus comes from the Latin word for poison. Unlike bacteria, viruses are not considered living things, because they cannot reproduce on their own — they need another living thing, such as an animal, a human, or even a plant, in order to reproduce. Bacteria, by contrast, have their own metabolic activity and replicate themselves without needing to invade a host.

Viruses are also many times smaller than bacteria. They cannot be seen with a regular microscope and require specialized equipment to view — the polio virus, for example, is about 10,000 times smaller than a grain of salt. Because they need little nutrition and are metabolically inactive, viruses can survive outside of a host.

A very simple structure

Viruses do not share the same basic cellular structure as other living things. A virus has a simple structure: just a cell envelope surrounding its genetic material, which can be either DNA or RNA. Because they lack the structures found in bacteria or human cells, viruses need a host in order to replicate.

How viruses infect a cell

Viruses have several ways to gain access to a host cell. Some attach to the cell and inject their reproductive protein into it; others merge with the cell; and still others create a channel through the cell wall. Once inside, the virus hijacks the host cell to make more viruses, which are then released to continue the process. The cells that produce the new viruses then die — which can trigger a reaction from the body, such as a fever or other illness.

Why antibiotics don't work on viruses

Antibiotics work against bacterial infections by disrupting specific cellular activities of the bacteria, killing them and curing the infection. Since viruses have no cellular activity of their own, there is nothing for an antibiotic to disrupt — which is why antibiotics are useless against viruses. There are certain antiviral drugs that work instead by disrupting the way a virus interacts with and takes over a host cell.

Prevention is the best medicine

Perhaps the best treatment for a viral infection is prevention — from avoiding infection in the first place to vaccines that prevent infection after exposure. Vaccines work by exposing the immune system to a non-infectious portion of a virus, allowing the body to develop the white blood cells needed to fight the real virus later. This is how the Hepatitis B vaccine works: small doses of parts of the virus help us build the fighter cells that will destroy it if we are ever exposed.

In short, viruses are very different from bacteria: they are not considered living tissue, they need a host cell to replicate, and antibiotics are ineffective against them.