The smallest living things are viruses. They are “alive” only because they can produce more of their kind if they invade another living thing. Viruses cannot reproduce on their own. They get into another living cell, the host cell, and take over its life processes to make more copies of themselves. In the process they destroy the host cell. A typical virus has an outer shell or coat made of proteins. Inside is a length of genetic material, either DNA or RNA, a single-strand version of DNA. Viruses are incredibly tiny: the smallest ones studied are 20 billionths of a metre in diameter. Most viruses are too small even to be observed through an optical microscope, and can only be viewed through electron microscopes. The study of viruses is called virology.

Kinds of virus

Different viruses are shaped like bricks, rods, golf balls and even space rockets. Many can exist in their non-living form for years and be frozen solid, boiled or made into crystals—yet still come alive when host cells are available.

Viruses cause diseases in plants, animals and people. These include colds, flu, measles and AIDS (caused by the Human Immunodeficiency Virus, HIV).

Bacteriophages

Bacteriophages, often known simply as "phages", are large viruses that attack bacteria. Each consists of a head, or capsid, containing a length of genetic material, a rod-like body contained in a sheath and a set of six slender tail fibres that anchor the phage to its host.

​​​​​​​Phages are found wherever bacteria exist. They are the most numerous organism on Earth, and are particularly densely concentrated in seawater. Scientists estimate there could be around 250 million phages in a millilitre of seawater. Up to 70% of marine bacteria may be infected by phages. Phages are seen as a possible treatment against drug-resistant strains (varieties) of many bacteria. This type of treatment is known as phage therapy.

Coronaviruses are a family of viruses that cause respiratory illnesses. These range from the common cold to more severe diseases such as Middle East Respiratory Syndrome (Mers), Severe Acute Respiratory Syndrome (Sars) and Covid-19, which became a pandemic in 2020. Symptoms include fever, coughing and breathing difficulties. Cases of severe infection can result in pneumonia and, in a few cases, lead to death.

The name coronavirus comes from the Latin corona, meaning "crown" or "halo", because of the resemblance of the virus—spherical and covered with club-shaped spikes—to the Sun and its corona, the shimmering outer atmosphere.

A coronavirus consists of a spike-covered, spherical protein capsule containing a strip of RNA (ribonucleic acid), its genetic material. After the coronavirus gets inside the human body, its spikes lock on to the surface of a cell lining the lungs. This allows the virus to break into the cell. Once inside, it takes over the cell’s ability to produce more copies of itself before breaking out of the cell again—killing it as it does so.

​​​​​​​In December 2019, a pneumonia outbreak was reported in Wuhan, China. The outbreak was traced to a new strain of coronavirus, later named Sars-CoV-2. The coronavirus quickly spread to other parts of the world, causing 6.1 million deaths (as of April 2022, and likely to be higher still). Scientists found that the Wuhan strain has a 96% genetic similarity to a bat coronavirus. So it is widely suspected to have originated in bats and been transmitted, possibly via another mammal, to humans.