Virology

Virology is the scientific study of biological viruses. It is a part of microbiology that looks at how viruses are detected, their structure and classification, how they change over time, and how they infect and use host cells to reproduce. The field also studies how viruses interact with the body's immune system, the diseases they cause, and ways to grow and study them in labs, as well as their uses in research and medicine.

The field of virology began when Martinus Beijerinck discovered that a disease in tobacco plants was caused by something new — not a bacterial or fungal infection, but a tiny agent he called a "virus." He described it as a 'contagious living fluid.' Later, Rosalind Franklin revealed the full structure of the tobacco mosaic virus in 1955.

Studying viruses is important because they cause many diseases in plants, animals, and humans. Scientists who study how viruses cause disease are learning about viral pathogenesis and the virulence — or how strong the disease caused by a virus can be. There are special areas of study just for viruses that infect plants (plant virology), animals (animal virology), and humans (medical virology).

Today, scientists have many tools to see and study viruses and their parts. Thousands of different viruses are now known, and virologists often focus on viruses that infect plants, microorganisms, or animals. Virology touches on many areas, including biology, health, animal welfare, farming, and ecology.

History

Main articles: History of virology and Social history of viruses

In the late 1800s, scientists began to discover tiny germs that were too small to see with regular microscopes. One scientist, Martinus Beijerinck, showed that a sickness in tobacco plants was caused by a special kind of germ he called a virus. This was the start of the science of virology.

Later, scientists learned how to grow viruses in labs using cells from animals and plants. They also discovered many new viruses, including ones that make animals and plants sick. Tools like the electron microscope helped scientists see viruses for the first time and learn about their shapes and structures.

Detecting viruses

Scientists have many ways to find viruses. One way is to look for tiny parts of the virus called antigens or for the virus’s genetic material. They also test whether a virus can still cause infection.

Electron microscopes help scientists see viruses. These microscopes use beams of electrons instead of light, allowing them to see much smaller objects than regular microscopes. A special method called negative staining makes viruses stand out against a dark background. Another method, cryogenic electron microscopy, keeps viruses safe in icy water so scientists can study their shapes very clearly.

To grow viruses in a lab, scientists use living cells from animals, bacteria in test tubes, or plants. When viruses grow in these cells, they can change the cells in ways that help scientists identify them. Scientists also use special tests with antibodies — proteins that recognize viruses — to study and detect them. One common test is called PCR, which looks for tiny bits of the virus’s genetic material. These tests are very good at finding viruses but need careful handling to avoid mistakes.

Quantitation and viral loads

Main article: Virus quantification

Counting viruses is very important in virology, especially for controlling some human infections where the viral load is measured. There are two main ways to count viruses: one counts only the viruses that can infect cells, called infectivity assays, and the other counts all virus particles, including those that cannot infect.

Infectivity assays measure how many viruses can infect cells. They use either live cells, plants, or lab-grown cells to see how many viruses are present. Some tests give exact numbers, while others show the chance that a sample will cause infection. For bacteria viruses, called bacteriophages, scientists can count them by looking at holes they make in a layer of bacteria. Another method uses special stains to find infected cells without waiting for visible spots to form. These tests can quickly show how many viruses are able to infect.

Viral load assays count the total number of virus genes instead of just the infectious ones. These tests are very useful for tracking diseases like HIV and can also be used for viruses that affect plants. They often use a process called PCR to find and count the virus genes.

Main article: Viral load

Molecular biology

Molecular virology studies viruses at the level of their genetic material and proteins. Scientists use special tools and methods to learn about viruses, and because viruses are small and simple, they are great subjects for these studies.

Viruses need to be cleaned and separated from other materials before scientists can study them. One way to do this is by using machines called centrifuges. High-speed centrifuges can separate viruses from heavier substances, making it easier to study them. Another method uses electric charge to separate virus parts, which helps scientists see and understand them better.

Sequencing helps scientists read the genetic code of viruses, which is important for learning how viruses cause disease and how to fight them. By studying these codes, scientists can track how viruses change and spread. Special techniques also let scientists make copies of virus parts without needing the actual virus, which is useful for creating tests and vaccines. Viruses that infect bacteria, called bacteriophages, have been especially helpful in research because they are easy to grow and study in labs.

Genetics

All viruses have genes, and scientists study these genes using tools from molecular biology, like cloning and creating changes called mutations such as RNA silencing.

One way scientists study virus genes is through reassortment, where genes from different viruses mix, especially in viruses with segmented genomes like influenza viruses and rotaviruses. Another method is recombination, where smaller pieces of DNA or RNA join together during replication. Scientists also use a technique called reverse genetics to create modified viruses for research.

Virus classification

Main article: Virus classification

Virus classification is a big part of virology, the study of viruses. Scientists group viruses based on shared features, not on the animals or plants they infect. In 1962, researchers André Lwoff, Robert Horne, and Paul Tournier created the first system to classify viruses, using a method similar to how living things are grouped. This system looks at things like the type of genetic material viruses have.

Today, the International Committee on Taxonomy of Viruses (ICTV) manages virus classification. They use a detailed system with many levels, from large groups called "realms" down to specific types called "species." This helps scientists understand the many different kinds of viruses.

The Baltimore classification, named after scientist David Baltimore, groups viruses based on how they make the molecules needed to build new viruses. This system divides viruses into seven main groups depending on whether their genetic material is DNA or RNA, and whether it is single-stranded or double-stranded. Examples include adenoviruses, coronaviruses, and retroviruses.