Because bacteria and viruses cause many of the diseases we're familiar with, people often confuse these two microbes. But viruses are as different from bacteria as goldfish are from giraffes.
For one thing, they differ greatly in size. The biggest viruses are only as large as the tiniest bacteria.
Another difference is their structure. Bacteria are complex compared to viruses.
A typical bacterium has a rigid cell wall and a thin, rubbery cell membrane surrounding the fluid, or cytoplasm (sigh-toe-plasm), inside the cell. A bacterium contains all of the genetic information needed to make copies of itself—its DNA—in a structure called a chromosome (crow-moe-soam). In addition, it may have extra loose bits of DNA called plasmids floating in the cytoplasm. Bacteria also have ribosomes (rye-bo-soams), tools necessary for copying DNA so bacteria can reproduce. Some have threadlike structures called flagella that they use to move.
A virus may or may not have an outermost spiky layer called the envelope. All viruses have a protein coat and a core of genetic material, either DNA or RNA. And that's it. Period.
Which brings us to the main difference between viruses and bacteria—the way they reproduce.
Bacteria contain the genetic blueprint (DNA) and all the tools (ribosomes, proteins, etc.) they need to reproduce themselves.
Viruses are moochers. They contain only a limited genetic blueprint and they don't have the necessary building tools. They have to invade other cells and hijack their cellular machinery to reproduce. Viruses invade by attaching to a cell and injecting their genes or by being swallowed up by the cell.
Here's an example of viral infection. This is virus version of the horror movie Alien.
These are T4 bacteriophages (back-tear-e-oh-faj-es). They are a kind of virus that infects bacteria. Here they are landing on the surface of an E. coli bacterium.
The bacteriophage cuts a hole in the E. coli's cell wall. It then injects its genetic material into the bacterium. By taking over the E. coli's genetic machinery, the viral genes tell the bacterium to begin making new virus parts. These parts come together to make whole new viruses inside the bacterium.
Eventually so many new viruses are made that the E. coli bursts open and dies, releasing all those new viruses to infect more cells!