Bacteria are living single-celled organisms that feed, grow and reproduce on their own — the ones in yoghurt and in a wound infection. A virus is not a living cell at all: it is genetic material in a protein coat that can only replicate by taking over a host's cells, as flu and the common cold do. The practical consequence: antibiotics kill bacteria and do nothing to a virus.
The core difference
A bacterium is a complete cell. It has its own machinery for producing energy and copying itself, and many species live perfectly well outside any host — in soil, in water, on your skin. A virus has no such machinery. Outside a cell it is inert; it reproduces only by entering a host cell and using that cell's equipment to make copies of itself.
Size follows from that. Bacteria are typically a few micrometres across and visible under an ordinary light microscope. Most viruses are tens to hundreds of nanometres — far smaller, and generally requiring an electron microscope.
The test you can actually apply
Ask whether antibiotics treat it. If the answer is yes, it is bacterial. Strep throat, bacterial pneumonia, urinary tract infections and tetanus are bacterial. Colds, flu, chickenpox, measles, norovirus and COVID-19 are viral, and antibiotics will not touch them. This is why a doctor may decline to prescribe antibiotics for a sore throat — not to ration them arbitrarily, but because they do nothing against a virus.
Vaccines, by contrast, exist for both kinds of infection, so the presence of a vaccine tells you nothing either way.
Getting the grammar right
- Bacteria is plural. The singular is bacterium: "A single bacterium divides into two."
- Bacterias is not a standard word. Neither is treating bacteria as singular in careful writing — "the bacteria are resistant", not "is resistant", though the singular use does turn up in speech.
- Virus is straightforward: the plural is viruses. There is no "viri" or "virii" in English.
Where the two get conflated
The word germ covers both, and so does bug in everyday speech — which is why people say "I've picked up a bug" without knowing which they have. That looseness is fine in conversation but unhelpful when the treatment differs.
Another point often missed: most bacteria are harmless or actively useful. Your gut depends on them, cheese and yoghurt are made with them, and soil bacteria recycle nutrients. Viruses have no comparable everyday benign role in the body, although harmless infections are common and some viruses are used deliberately in medicine and research.
The mistake worth avoiding
Do not describe an antibiotic as an "anti-virus" or expect leftover antibiotics to shorten a cold. Antiviral medicines are a separate class, they exist for only some viruses, and they generally work best when started early.
Frequently Asked Questions
Do antibiotics work on a virus?
No. Antibiotics target structures and processes that bacteria have and viruses do not, so they have no effect on a viral infection such as a cold or flu. Antivirals are a different class of drug and exist for only some viruses.
What is the singular of bacteria?
Bacterium. Bacteria is the plural, so "the bacteria are multiplying" is correct in careful writing. "Bacterias" is not standard English.
Is a virus alive?
Most biologists say no, or at least not fully. A virus cannot metabolise or reproduce by itself and is inert until it enters a host cell, which is why it sits on the boundary of the usual definitions of life.