Virus

A virus is a tiny living thing that can only grow by taking over the cells of another organism. Viruses can infect all kinds of life, including animals, plants, bacteria, and even other tiny organisms. They are found almost everywhere on Earth and are the most common type of biological entity. Scientists have described more than 16,000 different virus species, but there are likely millions more that we don’t fully understand yet.

When a virus gets inside a cell, it uses that cell’s machinery to make many copies of itself. Outside of a cell, a virus looks like a small particle made of genetic material wrapped in a protein coat, sometimes with an extra layer of lipids. These particles are so small that you need special tools to see them.

Viruses spread in many ways. Some are carried by insects like aphids that feed on plant sap, while others move through the air when people cough or sneeze. Foods and water can also carry viruses, as can contact between people. Our bodies usually fight off viruses with an immune response, and vaccines can help prepare our immune systems to protect us better. Some viruses, however, can avoid these defenses and cause long-lasting infections.

Etymology

The word "virus" comes from an old Latin word meaning "poison." Long ago, people used this word to describe harmful liquids. Over time, it came to mean something that can make us sick. Today, we know that viruses are tiny things that can make living things ill.

Origins

See also: Viral evolution

Viruses have been around almost as long as life itself, probably existing since the first living cells appeared. We don't know exactly where they came from because they don't leave fossils behind. Scientists use special methods to study their history.

There are three main ideas about how viruses began. One idea is that viruses were once small cells that lived by taking over bigger cells. Over time, they lost the ability to live on their own. Another idea is that viruses came from tiny bits of genetic material that broke away from larger organisms. A third idea suggests that viruses evolved at the same time as the very first cells on Earth. Even though we have many clues, scientists are still figuring out the true origin of viruses.

Microbiology

Discovery

The first clues that viruses exist came from experiments using filters with very tiny holes that could catch bacteria. In 1892, Dmitri Ivanovsky used one of these filters to show that sap from a sick tobacco plant could still make healthy tobacco plants sick, even after being filtered. Martinus Beijerinck called this filtered, infectious substance a “virus,” and this discovery marked the beginning of virology. Later discoveries, including the finding of bacteriophages by Frederick Twort and Félix d’Herelle, helped advance the field, and by the early 20th century, many viruses had been discovered. In 1926, Thomas Milton Rivers described viruses as obligate parasites. Wendell Meredith Stanley showed that viruses are particles, not fluids, and the invention of the electron microscope in 1931 allowed scientists to see their complex structures.

Life properties

Scientists debate whether viruses are a form of life or simply structures that interact with living organisms. They are often called “organisms at the edge of life” because, like living things, they have genes, change through natural selection, and make copies of themselves. However, unlike living cells, viruses do not have their own metabolism and need a host cell to reproduce. They cannot reproduce on their own outside a host cell but can self-assemble within cells. This ability to self-assemble is important for understanding how life might have begun. The virocell model suggests that an infected cell acts as the “living form” of a virus, with virus particles similar to spores.

Structure

Viruses come in many sizes and shapes. They are much smaller than bacteria, and many viruses could fit inside a single bacterial cell. Most viruses studied are spherical, ranging from 20 to 300 nanometres in diameter. Some viruses, like filoviruses, can be much longer—up to 1400 nanometres. Because most viruses are too small to see with a regular microscope, scientists use electron microscopes to study them.

A complete virus particle, called a virion, has genetic material wrapped in a protein coat called a capsid. Some viruses also have a lipid envelope taken from the host cell’s membrane. The capsid’s shape helps classify viruses into different types, such as helical, icosahedral, prolate, enveloped, and complex. For example, the tobacco mosaic virus has a helical structure, while the influenza virus is enveloped.

Genome

Viruses have either DNA or RNA genetic material. Their genomes can be circular or linear and may be divided into segments. The size of a virus’s genome varies widely; the smallest have only two kilobases and code for two proteins, while the largest, like pandoraviruses, have genomes around two megabases and code for about 2,500 proteins. RNA viruses usually have smaller genomes than DNA viruses because they make more mistakes when copying their genetic material. To avoid these mistakes, some RNA viruses have segmented genomes, which split the genetic material into smaller pieces.

Replication cycle

Viruses cannot grow or divide like cells. Instead, they take over a host cell’s machinery to make copies of themselves. The process has six main steps: attachment, penetration, uncoating, replication, assembly, and release. During attachment, the virus binds to specific receptors on the host cell. It then enters the cell, removes its capsid, and uses the cell’s resources to replicate its genetic material and make new virus proteins. Finally, new virus particles are assembled and released from the host cell, sometimes killing the cell in the process.

Host range

Viruses are found everywhere and can infect animals, plants, bacteria, and fungi. Some viruses can only infect one type of host, like the smallpox virus, which only affects humans. Others, like the rabies virus, can infect many different animals. Plant viruses do not harm animals, and most animal viruses do not affect humans. The collection of all viruses in an organism or environment is called a virome. For example, the human virome includes all viruses that infect humans.

Novel viruses

A novel virus is one that has never been recorded before. It might be discovered in its natural habitat or when it infects a new host, such as an animal or human, where it hadn’t been seen before. The SARS-CoV-2 virus, which caused the COVID-19 pandemic, is an example of a novel virus.

Classification

Scientists group viruses by their similarities to help understand them better. In 1962, three scientists created one way to classify viruses. Later, in 1966, a group called the International Committee on Taxonomy of Viruses was formed to help organize viruses.

Today, there are many groups and types of viruses. Scientists use two main systems to classify viruses. One system looks at the type of genetic material viruses have and how they are organized. The other system looks at how viruses make important molecules to grow and spread. These systems help scientists study and talk about viruses clearly.

Main article: Baltimore classification

Role in human disease

Some viruses cause common illnesses like the common cold, flu, chickenpox, and cold sores. More serious diseases such as rabies, Ebola virus disease, AIDS (HIV), avian influenza, and SARS are also caused by viruses.

Viruses can make us sick in different ways, depending on the type of virus. They might break open cells, which can make the whole body feel unwell if enough cells are affected. Some viruses, like those that cause cold sores, can stay hidden in the body for long periods without making us feel sick. Other viruses can stay in the body for a lifetime, leading to ongoing health issues.

Infection in other species

Main articles: Animal virus and Veterinary virology

Main article: Plant virus

Main article: Bacteriophage

Main article: Archaeal virus

Viruses can infect many different kinds of living things, such as animals, plants, bacteria, and archaea. Each type of living thing usually only gets infected by certain viruses. Some small viruses, called satellites, need another virus to help them copy themselves inside a cell.

Viruses are important for animals we raise for food. Diseases like foot-and-mouth disease and bluetongue are caused by viruses. Pets like cats, dogs, and horses can also get serious infections if they are not protected. Canine parvovirus is a small virus that can make puppies very sick. Many viruses live inside their hosts without causing any harm or making them feel ill.

There are many kinds of viruses that infect plants. These usually only make plants produce less, but they do not harm people or animals because they can only live inside plant cells. The potato virus Y can hurt potato crops and is spread by aphids. Plants have good ways to fight off viruses, like special genes that stop the virus from spreading.

Bacteriophages are viruses that infect bacteria. They are very common in water and can be found in much larger numbers than bacteria themselves. These viruses attach to bacteria and make copies of themselves inside. Bacteria have ways to protect themselves, such as enzymes that break down the virus’s DNA and systems that remember past infections to stop new ones.

Some viruses that infect archaea—another type of tiny living thing—have unusual shapes. These tiny organisms also have ways to defend themselves against viruses, similar to how plants and bacteria do.

Role in aquatic ecosystems

Main article: Marine virus

Viruses are very common in water. There are about ten million of them in just a small amount of seawater. Many of these viruses infect tiny organisms and help control life in both salty and fresh water. They play an important role in recycling nutrients and carbon in the ocean. When viruses infect and break down tiny organisms, they release nutrients that help new life grow.

Most of the living things in the sea are tiny organisms, and viruses help control their numbers. Scientists have found that viruses are carried around the world in the air, landing on every part of our planet every day. Some viruses can also infect animals that live in the water.

Role in evolution

Main article: Horizontal gene transfer

Viruses help move genes between different kinds of living things. This mixing of genes makes life more varied and helps it change over time. Scientists believe viruses were very important in the very early days of life, long before all living things split into groups like bacteria and plants. Today, viruses still hold many genes that scientists have not yet studied.

Applications

Life sciences and medicine

Viruses are important for studying how cells work. Scientists use them to learn about genetics, how cells copy their DNA, and how they build proteins. Viruses can also help treat diseases. For example, some viruses can be changed to target and destroy cancer cells without harming healthy ones. This kind of treatment has been used for skin cancer and shows promise for other illnesses too.

Materials science and nanotechnology

In materials science, viruses are used like tiny building blocks. Their special shapes and sizes help scientists create new materials on a very small scale. For example, some viruses are used to improve sensors that can detect DNA.

Synthetic viruses

Scientists can now make viruses from scratch by creating their genetic code in a lab. This helps in developing new vaccines and studying how viruses work. Once the code is made, it can start a new virus when placed inside a cell.

Weapons

Viruses have caused big outbreaks in the past, raising concerns that they could be used as weapons. For example, the virus that caused the 1918 flu was recreated in a lab. Smallpox, a very dangerous virus, is kept only in two special labs for research. Because many people today are not protected against smallpox, it could be very harmful if used as a weapon.