A virus is a microscopic infectious agent that causes disease and can only reproduce inside the cells of living organisms. Viruses infect all organisms, from animals and plants to microorganisms like bacteria. The study of viruses is known as virology.
Viruses do not have any of the building blocks that are essential to all life, such as adenosine triphosphate (ATP), ribosomes and mitochondria. Because of this, they are not considered to be living. However, they do contain DNA or RNA and they can replicate using the replication machinery of the cells they infect. This makes them genetic parasites.
Typically, viruses consist of a protein shell, called a capsid, that covers a nucleic acid core. This core contains the viral genome, which is encoded in RNA or DNA. The virus’s ability to cause disease is related to the properties of this genetic material.
When a virus is ready to infect a cell, it releases a bunch of its proteins that can bind to receptors on the cell’s surface. The virus then inserts itself into the cell and its genetic information is released into the cytoplasm. The virus then uses the cell’s machinery to make copies of itself, which are then free to infect more cells.
Viruses can be shaped in many ways, from helically coiled to icosahedral (a shape with many sides, like a soccer ball). They may or may not have an envelope. The envelope, if present, is made of an envelope protein. Depending on the type of virus, it can also have spiky structures that help it bind to and enter cells.
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