A virus is an infectious microbe whose genetic material — either DNA or RNA — is surrounded by a protein shell, called a capsid. Viruses cannot replicate on their own; instead they invade host cells and use components of the cell to make more viruses. Viruses can cause disease in humans and animals, such as AIDS, COVID-19, measles and smallpox. They can also be used to deliver healthy genes to cells that are defective or diseased, a process known as gene therapy.
The study of viruses is known as virology. Viruses are very small, ranging in size from 10 to 100 times smaller than bacteria. Their size and shape have made it hard to study them until recently, when they were visualized using electron micrographs. Viruses contain nucleic acid, but they do not have the chemical machinery (enzymes) needed for life. Because of this, biologists don’t consider them to be alive. Think of a bean seed: it has the properties of life, but isn’t alive until planted in the soil.
When a virus is not inside an infected cell, it exists as an independent particle called a virion. The virion consists of a viral genome, surrounded by a proteins, and, in some cases, an outside envelope of lipids. The viral genome can be single-stranded RNA (ssRNA) or double-stranded DNA, and can vary in length. The virion can also have a helical or icosahedral structure, and the structure of its protein coat can be simple or complex.