Viruses cause colds and flu, so people often lump them in with bacteria. They are very different. A virus is not even a cell.
Key idea
A virus is genetic material (DNA or RNA) inside a protein coat. It has no membrane, cytoplasm or ribosomes of its own, so it can make copies of itself only inside a living host cell. Most viruses are about 20 to 300 nanometers across, far smaller than bacteria.
Why it works
A virus attaches to a host cell and gets its genetic material inside. The cell's own ribosomes and supplies then build new virus parts, which assemble into new viruses and leave the cell. Outside a cell, a virus does nothing at all: it does not eat, grow or use energy. That is why scientists still debate whether viruses count as alive.
Worked examples
Worked example: How small?
A bacterium is 2 µm long. A virus is 100 nm across. About how many such viruses fit along the bacterium? (1 µm = 1,000 nm)
- Bacterium: $2 \times 1000 = 2000$ nm.
- Viruses in a row: $2000 \div 100 = 20$.
Answer: About 20.
Worked example: Antibiotics?
Will an antibiotic act on a cold virus?
- Antibiotics damage parts of bacterial cells, such as the cell wall or ribosomes.
- A virus has no cell wall and no ribosomes of its own.
Answer: No. Antibiotics do not work on viruses.
Common mistake
Thinking viruses are tiny bacteria and that antibiotics work on them. Viruses are far smaller, are not cells, and antibiotics do not act on them (CDC). Check: does it have its own ribosomes? Bacteria do; viruses do not.
Vaccines work by training the immune system to remember a virus, so it can respond quickly later. How the body defends itself comes in the Human Body unit.