Virus

A virus is a tiny living thing that can only grow by taking over the cells of another organism. Viruses can infect animals, plants, bacteria, and other tiny organisms. They are found almost everywhere on Earth and are very common. Scientists have described many different virus species, but there are probably many more we don’t know about yet.

When a virus gets inside a cell, it uses that cell 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.

Etymology

The word "virus" comes from an old Latin word meaning "poison." Long ago, people used this word to describe harmful liquids. Now, we know that viruses are tiny things that can make us sick.

Origins

See also: Viral evolution

Viruses have been around almost as long as life itself. They probably existed when 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. Scientists are still figuring out the true origin of viruses.

Microbiology

Discovery

The first clues that viruses exist came from experiments with very tiny filters. In 1892, Dmitri Ivanovsky showed that sap from a sick tobacco plant could still make healthy plants sick, even after being filtered. Martinus Beijerinck called this filtered substance a “virus,” and this discovery started the study of viruses. Later discoveries helped scientists learn more about them.

Life properties

Scientists wonder if viruses are a form of life or just structures that work with living things. They are called “organisms at the edge of life” because, like living things, they have genes, change through natural selection, and can make copies of themselves. But unlike living cells, viruses need a host cell to reproduce. They cannot reproduce on their own but can build themselves inside cells. This helps us understand how life might have begun. The virocell model suggests that an infected cell acts like the “living form” of a virus.

Structure

Viruses come in many sizes and shapes. They are much smaller than bacteria, and many could fit inside a single bacterial cell. Most viruses studied are round, ranging from 20 to 300 nanometres in diameter. Some viruses 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 from the host cell’s membrane. The shape of the capsid helps classify viruses into different types. 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 in pieces. The size of a virus’s genome varies; the smallest have only two kilobases, while the largest have genomes around two megabases. RNA viruses usually have smaller genomes than DNA viruses.

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. The virus binds to the host cell, enters it, uses the cell’s resources to make new virus parts, and then releases new virus particles, sometimes killing the cell.

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. 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, the International Committee on Taxonomy of Viruses was formed to help organize them.

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. The other system looks at how viruses grow and spread. These systems help scientists study viruses.

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 can stay hidden in the body for long periods without making us feel sick. Other viruses can stay in the body for a long time, leading to health problems.

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 kinds of living things, like animals, plants, bacteria, and archaea. Each kind of living thing usually only gets sick from certain viruses. Some small viruses, called satellites, need help from another virus to copy themselves.

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

There are many kinds of viruses that infect plants. These usually only make plants grow 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 spreads by aphids. Plants have good ways to fight off viruses.

Bacteriophages are viruses that infect bacteria. They are very common in water and can be found in larger numbers than bacteria. These viruses attach to bacteria and make copies of themselves inside. Bacteria have ways to protect themselves.

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.

Role in aquatic ecosystems

Main article: Marine virus

Viruses are very common in water. There are many of them even in a small amount of seawater. Many of these viruses infect tiny organisms and help control life in both salty and fresh water. They help recycle nutrients and carbon in the ocean. When viruses infect and break down tiny organisms, they release nutrients that help new life grow.

Most living things in the sea are tiny organisms, and viruses help control their numbers. Scientists have found that viruses can travel around the world in the air, landing on every part of our planet each 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 a long time ago, before life split into groups like bacteria and plants. Today, viruses still carry many genes that scientists are still learning about.

Applications

Life sciences and medicine

Viruses help scientists learn how cells work. They study genetics, how cells copy their DNA, and how they build proteins. Some viruses can be changed to fight cancer by targeting cancer cells and leaving healthy cells unharmed. This treatment has helped with skin cancer and might help with other illnesses too.

Materials science and nanotechnology

In materials science, viruses act like tiny building blocks. Their unique shapes and sizes help scientists make new materials on a very small scale. For example, some viruses help improve sensors that can detect DNA.

Synthetic viruses

Scientists can now create viruses in a lab by building their genetic code. This helps in making new vaccines and understanding how viruses work. Once the code is made, it can start a new virus inside a cell.

Weapons

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