virus

The word “virus” means a microscopic infectious agent that can only reproduce inside a living host cell, sometimes causing disease. In plain terms, a germ that causes disease, or, separately, malicious computer software that spreads and causes harm.

“Virus” is pronounced VEYE-ruhs (/ˈvaɪ.ɹəs/ in IPA).

Here is “virus” used in a sentence: “The flu virus mutates constantly, which is why new vaccines are needed every year.”

“Virus” is a noun.

The word “virus” dates back to around 1972 and comes from Latin. From Latin virus, meaning "poison" or "slimy liquid." The computer sense was popularized after appearing in David Gerrold's 1972 novel When HARLIE Was One.

“Virus” is a fairly common word in modern English, usually learned at around the B1 level.

“Virus” has 3 distinct senses listed in this dictionary. The most common is: a microscopic infectious agent that can only reproduce inside a living host cell, sometimes causing disease.

Bacteria are living, single-celled organisms that can grow and reproduce on their own. A virus is far smaller and is not fully alive — it can only replicate by hijacking a living host cell. Crucially, antibiotics work against bacteria but have no effect whatsoever on viruses.

Scientists debate this. Viruses carry genetic material and can evolve, but they cannot grow, produce energy, or reproduce by themselves — they must invade a host cell to make copies. Because they lack the machinery of life on their own, many biologists place them on the boundary between living and non-living.

No. Antibiotics target bacteria and do nothing to viruses. Most viral infections are fought off by the body's own immune system, sometimes helped by antiviral medicines or prevented by vaccines. Taking antibiotics for a virus is ineffective and fuels antibiotic resistance, which is why doctors avoid prescribing them for it.

Viruses cause many familiar illnesses, including the common cold, influenza (flu), COVID-19, measles, chickenpox, and HIV. Some viral infections are mild and pass quickly, while others are serious or lifelong. Vaccines exist to prevent many of them, which is why immunisation programmes so often target viral diseases.