Virus

Viruses are very tiny.

They are too small to see. They live inside other living things. They can live in plants or animals.

A virus needs a cell to make more of itself. It goes inside a cell. Then, the cell makes many new copies.

Viruses can spread through the air. They can also move from person to person. Some viruses even move on insects. They are found almost everywhere on Earth.

A virus is a tiny agent that can make things sick.

They are much smaller than most bacteria. You cannot see them with a regular microscope. They can infect all life, like plants and animals.

To make more of itself, a virus must enter a living cell. This cell is called a host. Once inside, the virus forces the host to make many copies. A single virus particle is called a virion. A virion has two main parts. First, it has genetic material, like DNA or RNA. This is a set of instructions. Second, it has a protein coat called a capsid. This coat protects the instructions.

Viruses spread in many ways. Some spread through the air when people cough or sneeze. Others move through insects, like aphids that feed on plants. Some viruses move through food or water. Scientists study viruses in a field called virology. They want to know how these tiny things work and where they came from.

A virus is a tiny infectious agent that lives inside other living things.

These agents can infect almost all life forms, including animals and plants. They even infect tiny organisms like bacteria and archaea. Viruses are found in nearly every ecosystem on our planet. They are actually the most numerous type of biological entity in the world. Because they are so small, they are called submicroscopic.

To grow, a virus must enter a living cell called a host. Once inside, the host cell is often forced to make thousands of copies of the virus.

A single virus particle is called a virion. A virion usually has genetic material, which is DNA or RNA. This material contains the instructions for how the virus works. A protein coat called a capsid surrounds and protects this material.

Some viruses also have an outside envelope made of lipids.

Scientists first discovered viruses by using special filters. In 1892, Dmitri Ivanovsky showed that sap from sick tobacco plants stayed infectious after filtering.

In 1898, Martinus Beijerinck described the tobacco mosaic virus. This work helped start the study of viruses, which is called virology. Later, in 1931, the invention of the electron microscope helped scientists see their shapes. Before this, most viruses were too small for regular microscopes to see.

Viruses spread to new hosts in many different ways. Some spread through the air when people cough or sneeze. This includes viruses like influenza, measles, and chickenpox.

Other viruses move through insects, which are called vectors. For example, aphids can carry viruses from one plant to another. Some viruses, like norovirus, spread through contaminated food or water.

The amount of virus needed to cause sickness is sometimes very small. For norovirus, fewer than 100 particles can cause an infection.

Scientists still debate if viruses are truly alive. They have genes and they evolve, but they lack a cell structure.

This makes them "organisms at the edge of life." They do not have their own metabolism to create energy. Instead, they rely entirely on the host cell to function. This makes them very different from the cells in your own body. They are unique pieces of nature that bridge the gap between life and non-life.

A virus is a submicroscopic infectious agent that can only replicate inside the living cells of an organism.

They are incredibly widespread, infecting all life forms, including animals, plants, and microorganisms like bacteria and archaea. Because they exist in almost every ecosystem on Earth, they are considered the most numerous type of biological entity. The study of these agents is called virology, which is a specialized branch of microbiology.

Scientists have described more than 16,000 specific virus species, though millions more likely exist.

When a virus is not inside a host, it exists as an independent particle called a virion.

A virion typically consists of three main parts. First, it contains genetic material in the form of long DNA or RNA molecules. These molecules encode the instructions for the virus's protein structures. Second, the genetic material is protected by a protein coat known as a capsid. In some cases, a third layer called an envelope, made of lipids, surrounds the capsid.

These particles come in many shapes, ranging from simple helical forms to complex icosahedral structures.

To reproduce, a virus must undergo a process of hijacking a host cell. Once the virus enters a host, it often forces that cell to rapidly produce thousands of new copies of the original virus.

The variety of cells a virus can infect is known as its host range. Some viruses have a narrow range, meaning they only infect a few specific species. Others have a broad host range and can infect many different types of organisms. In animals, these infections usually trigger an immune response to eliminate the virus. However, some viruses, such as HIV or those causing hepatitis, can evade the immune system to create chronic infections.

Viruses use many different pathways to spread between hosts. Some are transmitted by vectors, which are disease-bearing organisms like insects. For example, aphids can carry viruses between plants, while blood-sucking insects can carry viruses in animals. Other viruses spread through the air via coughing or sneezing, such as influenza, measles, and SARS-CoV-2.

Some viruses, like norovirus, are transmitted through the faecal–oral route via contaminated food or water. For norovirus, the infectious dose is remarkably small, requiring fewer than 100 particles to cause infection in humans.

Others, like HIV, are transmitted through sexual contact or exposure to infected blood.

Our understanding of viruses grew through key scientific discoveries. In 1892, Dmitri Ivanovsky used filters to show that sap from diseased tobacco plants remained infectious even after bacteria were removed.

In 1898, Martinus Beijerinck identified the tobacco mosaic virus, marking the beginning of virology. Later, Wendell Meredith Stanley proved that viruses were particles rather than fluids. The invention of the electron microscope in 1931 was a major turning point, as it allowed scientists to finally visualize these complex structures.

Before this, most viruses were too small to be seen with an optical microscope.

There is an ongoing scientific debate regarding whether viruses are truly alive. They are often called "organisms at the edge of life" because they possess certain biological traits. They carry genetic material, they evolve through natural selection, and they reproduce by self-assembly. However, they lack a cellular structure and do not have their own metabolism to produce energy. Unlike most living things that use cell division, viruses rely entirely on the host's machinery. Some scientists propose the virocell model, suggesting the infected cell itself is the living form of the virus.

Because viruses do not leave fossils, their exact origins remain a mystery. Scientists use molecular techniques to explore three main hypotheses. The regressive hypothesis suggests viruses were once small cells that lost genes as they became parasites. The cellular origin hypothesis proposes they escaped from the genes of larger organisms, perhaps from pieces of DNA called plasmids or transposons. The co-evolution hypothesis suggests they evolved alongside the very first cells on Earth.

It is likely that viruses have arisen many different times through various mechanisms throughout history.