A virus is a microscopic particle that can infect the cells of a biological organism.
Viruses can only replicate themselves by infecting a host cell and therefore cannot reproduce on their own.
At the most basic level, viruses consist of genetic material contained within a protective protein coat called a capsid; the existence of both genetic material and protein distinguishes them from other virus-like particles such as prions and viroids.
They infect a wide variety of organisms: both eukaryotes (animals, fungi and plants) and prokaryotes (bacteria).
A virus that infects bacteria is known as a bacteriophage, often shortened to phage.
The study of viruses is known as virology, and those who study viruses are known as virologists.
It has been argued extensively whether viruses are living organisms.
Most virologists consider them non-living, as they do not meet all the criteria of the generally accepted definition of life.
They are similar to obligate intracellular parasites as they lack the means for self-reproduction outside a host cell, but unlike parasites, viruses are generally not considered to be true living organisms.
A primary reason is that viruses do not possess a cell membrane or metabolise on their own - characteristics of all living organisms.
Examples of common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores.
Serious diseases such as Ebola, AIDS, bird flu and SARS are all also caused by viruses.