Virus vs bacteria: what's the difference?

The difference between a virus and bacteria is that bacteria are complete living cells, able to feed, grow and divide on their own, while a virus is a tiny package of genes in a protein coat that can only reproduce by taking over another cell. Bacteria are usually 10 to 100 times longer than viruses. That difference in how they live is why antibiotics kill bacteria but do nothing to a virus.

In short

• A bacterium is a single cell with its own machinery for making proteins and energy. A virus has none and borrows a host cell's.
• Most bacteria are harmless or useful, and your body carries about as many bacterial cells as human ones.
• Antibiotics attack parts that bacteria have and viruses lack, so they don't treat colds or flu.

How are viruses and bacteria different, side by side?

The two are often lumped together as "germs" because both can make people ill, but biologically they are about as different as a bicycle and a bicycle's instruction manual. A bacterium is a working machine. A virus is a set of instructions that needs someone else's factory.

	Bacteria	Viruses
What it is	A single living cell	Genes wrapped in a protein coat, sometimes with an outer envelope
Typical size	0.5–5 micrometers long	Mostly 20–300 nanometers across; rare giant viruses are larger
Seen with	An ordinary light microscope	Usually an electron microscope
Genetic material	DNA, both a main chromosome and often small rings called plasmids	DNA or RNA, depending on the virus
Own machinery	Ribosomes to build proteins, a metabolism to make energy	None of its own
How it reproduces	Splits in two (binary fission)	Hijacks a host cell to assemble new copies
Can live outside a host	Yes, in soil, water, food and on surfaces	It can survive for a time but can't multiply
Examples of disease	Strep throat, urinary tract infections, tuberculosis, cholera	Common cold, flu, measles, COVID-19, HIV
Treated by	Antibiotics, for many infections	Antivirals for some; vaccines prevent many

A useful sense of scale: more than a thousand bacteriophages, the viruses that infect bacteria, would fit inside one Escherichia coli cell. Viruses infect every form of life, including bacteria themselves.

How does each one make copies of itself?

A bacterium reproduces by growing, copying its DNA and splitting into two identical daughter cells. In good conditions some kinds double as fast as every 17 minutes. That speed is easy to underestimate, so here it is worked through.

A worked example: one cell, one working day

Suppose a single bacterium lands in a warm, nutrient-rich spot and divides every 20 minutes. After one hour there are 8 cells (three doublings: 2, 4, 8). After four hours, twelve doublings, there are 4,096. After eight hours, 24 doublings, there are 2²⁴, which is about 16.8 million. Real colonies slow down long before that pace runs away, as food runs out and waste builds up, but the numbers show why food left in the warm for an afternoon can carry far more bacteria than food fresh from the fridge.

A virus works quite differently. It drifts until it meets a cell with the right receptor on its surface, the lock its proteins fit. It gets its genes inside, and the cell's own machinery starts reading them as if they were the cell's instructions, building virus parts instead of its own. New virus particles assemble inside, often hundreds or thousands from a single cell, and then burst out or bud off to infect neighbors. Outside a cell, a virus does nothing at all: it has no metabolism and doesn't grow.

Are viruses alive?

Bacteria clearly are: they take in food, produce energy, respond to their surroundings and reproduce on their own. Viruses sit on the border, and biologists still disagree about which side.

Still argued

Whether viruses count as living is a question of definition, not of facts in dispute. Those who say yes point out that viruses carry genes, reproduce and evolve by natural selection, as anything alive does. Those who say no point out that they have no cells, no metabolism, and no way to do anything outside a host. The discovery of giant viruses, some larger than the smallest bacteria and carrying genes once thought unique to cells, has kept the debate going.

Why do antibiotics work on bacteria but not viruses?

Antibiotics are drugs that target structures bacteria have and human cells don't. Penicillin, made by a mold, blocks bacteria from building their cell walls from a material called peptidoglycan; other antibiotics jam bacterial ribosomes, which differ from human ones. A virus has no cell wall, no ribosomes and no metabolism of its own, so there is nothing for an antibiotic to hit.

The U.S. Centers for Disease Control and Prevention states it plainly: antibiotics do not work on viruses, including colds and runny noses, even when the mucus is thick, yellow or green, most sore throats (strep throat is the exception), the flu and most chest colds. Taking antibiotics for a viral illness doesn't help and can cause side effects.

It also has a cost for everyone. Every course of antibiotics gives the bacteria that happen to survive it a chance to multiply, which is how resistance spreads. The World Health Organization's fact sheet on antimicrobial resistance, dated 16 July 2026, cites an estimate that bacterial resistance was associated with more than 4.7 million deaths worldwide in 2021.

Viral infections have their own defenses. Antiviral drugs block particular steps in a virus's copying, as medicines for HIV, influenza and hepatitis C do. Vaccines train the immune system in advance, and they exist for both viral diseases, such as measles, and bacterial ones, such as tetanus and whooping cough.

Are most bacteria harmful?

No. Only a small share of bacterial species cause disease in people. Most live in soil, water and other organisms, recycling nutrients, and many are essential: bacteria in the gut help digest food and make vitamins, and bacteria in legume roots turn nitrogen from the air into fertilizer.

The human body is full of them. A widely cited 2016 study by Ron Sender, Shai Fuchs and Ron Milo estimated that a 70 kg "reference man" carries about 38 trillion bacteria, against about 30 trillion human cells, weighing about 0.2 kg in all. The older claim that bacteria outnumber human cells ten to one was, by that study's count, a large overestimate.

Viruses aren't all enemies either. Phages kill vast numbers of bacteria in the oceans every day, and doctors have used them experimentally to treat bacterial infections that resist antibiotics.

How can you tell which one is making someone ill?

Often you can't tell from symptoms alone. Pneumonia, meningitis, ear infections and diarrhea can each be caused by either. A sore throat is usually viral but sometimes bacterial, as with strep. Clinicians decide with an examination and, where needed, a test: a throat swab, a urine culture, a blood test. This note explains biology, not medicine; anyone worried about an infection should see a doctor or other clinician rather than guess.

Telling viruses and bacteria apart at all was a feat of careful experiment. In 1892 Dmitri Ivanovsky showed that sap from diseased tobacco plants stayed infectious after passing through a filter fine enough to trap bacteria; in 1898 Martinus Beijerinck argued it held a new kind of agent. That agent, tobacco mosaic virus, was the first virus identified, decades before anyone could see one.

Check yourself

• Without looking back: name two things a bacterium has that a virus lacks, and explain why that matters for antibiotics.
• Why does taking antibiotics for a cold make resistance more likely, even though it doesn't treat the cold?
• Walk through what happens between a virus touching a cell and new viruses leaving it.

Sources

• Antibiotic Use: About — U.S. Centers for Disease Control and Prevention
• Antimicrobial resistance, fact sheet — World Health Organization, 2026
• Revised Estimates for the Number of Human and Bacteria Cells in the Body — Sender, Fuchs and Milo, PLOS Biology, 2016
• Virus — Wikipedia
• Bacteria — Wikipedia