A virus is a small, infectious agent that consists of a core of genetic material (either deoxyribonucleic acid [DNA] or ribonucleic acid [RNA]) surrounded by a shell of protein. Viruses cause disease by infecting a host cell and commandeering the host cell's synthetic capabilities to produce more viruses. The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the host.
Viruses can infect plants, bacteria, and animals. The tobacco mosaic virus, one of the most studied of all viruses, infects tobacco plants. Bacterial viruses, called bacteriophages, infect a variety of bacteria, such as Escherichia coli, a bacteria commonly found in the human digestive tract. Animal viruses cause a variety of diseases. Acquired immune deficiency syndrome (AIDS) is caused by the human immunodeficiency virus (HIV); hepatitis and rabies are viral diseases; and the so-called hemorrhagic fevers, which are characterized by severe internal bleeding, are caused by filoviruses. Zika virus disease is caused by a flavivirus and is transmitted most often by the bite of an infected mosquito. Other animal viruses cause some of the most common human diseases. Often, these diseases strike in childhood. Measles, mumps, and chickenpox are viral diseases. The common cold and influenza are also caused by viruses. Finally, some viruses can cause cancer and tumors. One such virus, human T-cell leukemia virus (HTLV), is associated with a form of leukemia in adults.
Beginning in December 2019, a small cluster of cases of a pneumonia of unknown origin was reported in Wuhan, Hubei Province, China. Genetic sequencing of samples taken from the respiratory tract secretions of those exhibiting the suspicious pneumonia revealed the causative agent as a previously unknown coronavirus, subsequently named the 2019-nCoV virus. On March 11th, the World Health Organization (WHO) officially declared COVID-19 a global pandemic. The virus was officially named SARS-CoV-2 (also styled SARS-CoV2). The disease associated with the virus is COVID-19 (also styled as Covid-19).
Background and Scientific Foundations
Structure of viruses
Although viral structure varies considerably between the different types of viruses, all viruses share some common characteristics. All viruses contain either RNA or DNA surrounded by a protective protein shell called a capsid. Some viruses have a double strand of DNA, others a single strand of DNA. Other viruses have a double strand of RNA or a single strand of RNA. The size of the genetic material of viruses is often quite small. Compared to the estimated 24,000 genes that exist within human DNA, viral genes number from 10 to about 200 genes.
The capsid of viruses is relatively simple in structure, owing to the few genes that the virus contains to encode the capsid. Most viral capsids consist of a few repeating protein subunits. The capsid serves two functions: it protects the viral genetic material, and it helps the virus introduce itself into the host cell. Many viruses are extremely specific, targeting only certain cells within the plant or animal body. When the virus encounters the correct receptors on a host cell, it docks with this host cell and begins the process of infection and replication.
Most viruses are rod-shaped or roughly sphere-shaped but poxviruses are rectangular, looking somewhat like bricks. Parapoxviruses are ovoid. Bacteriophages are the most unusually shaped of all viruses. A bacteriophage consists of a head region attached to a sheath. Protruding from the sheath are tail fibers that dock with the host bacterium. Bacteriophage structure is eminently suited to the way it infects cells. Instead of the entire virus entering the bacterium, the bacteriophage injects its genetic material into the cell, leaving an empty capsid on the surface of the bacterium.
All viruses consist of genetic material surrounded by a capsid, but variations exist within this basic structure. Studding the envelope of these viruses are protein spikes. These spikes are clearly visible on some viruses, such as the influenza viruses; on other enveloped viruses, the spikes are extremely difficult to see. The spikes help the virus invade host cells. The influenza virus, for instance, has two types of spikes. One type, composed of hemagglutinin protein, fuses with the host cell membrane, allowing the virus particle to enter the cell. The other type of spike, composed of the protein neuraminidase, helps the newly formed virus particles to bud out from the host cell membrane.
Viruses contain such small amounts of genetic material because the only activity that they perform independently of a host cell is the synthesis of the protein capsid. In order to reproduce, a virus must infect a host cell and take over the host cell's synthetic machinery.
Viruses are obligate intracellular parasites, meaning that in order to replicate, they need to be inside a host cell. Viruses lack the machinery and enzymes necessary to reproduce; the only synthetic activity they perform on their own is to synthesize their capsids.
The infection cycle of most viruses follows a basic pattern. First, the virus docks with the host cell. The genetic material of the virus then enters the host cell, and the virus loses its capsid. Once inside the host cell, the viral RNA or DNA takes over the cellular machinery to make more viruses. Viral subunits are produced, which are then assembled to make whole viruses. Finally, the new viruses leave the host cell, either by exocytosis or by destroying the host cell.
Bacteriophages are unusual in that they can infect a bacterium in two ways (although other viruses may replicate in these two ways as well). In the lytic cycle of replication, the bacteriophage destroys the bacterium it infects. In the lysogenic cycle, however, the bacteriophage coexists with its bacterial host, and remains inside the bacterium throughout its life, reproducing only when the bacterium itself reproduces.
Types of viruses
Scientists have classified viruses according to the type of genetic material they contain. Broad categories of viruses include double-stranded DNA viruses, single-stranded DNA viruses, double-stranded RNA viruses, and single stranded RNA viruses.
Poxviruses are the most complex kind of viruses known. They have large amounts of genetic material and fibrils anchored to the outside of the viral capsid that assist in attachment to the host cell. Poxviruses contain a double strand of DNA.
Herpes viruses are enveloped, double-stranded DNA viruses. Of the more than 50 herpes viruses that exist, only eight cause disease in humans. These include the human herpes virus types 1 and 2 that cause cold sores and genital herpes; human herpes virus 3, or varicella-zoster virus (VZV), that cause chickenpox and shingles.
Adenoviruses are viruses that attack respiratory, intestinal, and eye cells in animals. More than 40 kinds of human adenoviruses have been identified. Eye illnesses caused by adenoviruses include conjunctivitis, an infection of the eye tissues, as well as a disease called pharyngoconjunctival fever, a disease in which the virus is transmitted in poorly chlorinated swimming pools.
Human papoviruses include two groups: the human papilloma viruses (HPV) and the polyomaviruses. Human papoviruses are the smallest double-stranded DNA viruses. The viruses cause several kinds of benign (non-cancerous) warts, including plantar warts (those that form on the soles of the feet) and genital warts. HPV is the most common sexually transmitted infection; almost half of all sexually active men and women eventually acquire at least one strain of HPV at some point in their lives.
Human papoviruses contain oncogenes, or genes that encode for growth factors that initiate the uncontrolled growth of cells. This uncontrolled proliferation of cells is called cancer. When the oncogenes within an epithelial cell are activated, they cause the epithelial cell to proliferate. In the cervix (the opening of the uterus), the cell proliferation manifests first as a condition called cervical neoplasia. In this condition, the cervical cells proliferate and begin to crowd together. Eventually, cervical neoplasia can lead to full-blown cancer.
The hepadnaviruses cause several diseases, including hepatitis B. Hepatitis B is a chronic, debilitating disease of the liver and immune system.
Parvoviruses are icosahedral, single-stranded DNA viruses that infect a wide variety of mammals. Each type of parvovirus has its own host. For instance, one type of parvovirus causes disease in humans; another type causes disease in cats; while still another type causes disease in dogs.
Orthomyxoviruses cause influenza (flu). This highly contagious viral infection can quickly assume epidemic proportions, given the right environmental conditions. An influenza outbreak is considered an epidemic when more than 10 percent of the population is infected. Antibodies that are made against one type of influenza virus are often ineffective against other types of viruses. For this reason, most people are susceptible to flu from season to season.
Paramyxoviruses are helical, enveloped, single-stranded RNA viruses that cause pneumonia, croup, measles, and mumps in children. A vaccine against measles and mumps has greatly reduced the incidence of these diseases in the United States.
Two filoviruses, Ebola virus and Marburg virus, are among the most lethal of all human viruses. Both cause severe fevers accompanied by internal bleeding, which eventually kills the victim. The fatality rate of Marburg is about 60 percent, whereas the fatality rate of Ebola virus approaches 90 percent. Both are transmitted through contact with body fluids. Marburg and Ebola also infect primates.
Rhabdoviruses are bullet-shaped, single-stranded RNA viruses. They are responsible for rabies, a fatal disease that affects dogs, rodents, and humans.
Retroviruses are unique viruses. They are double-stranded RNA viruses that contain an enzyme called reverse transcriptase. Within the host cell, the virus uses reverse transcriptase to make a DNA copy from its RNA genome. In all other organisms, RNA is synthesized from DNA. Cells infected with retroviruses are the only living things that reverse this process. Retroviruses include the human T-cell leukemia virus (HTLV) and human immunodeficiency virus (HIV), the virus responsible for acquired immunodeficiency syndrome (AIDS). Both HIV and HTLV are transmitted in body fluids.
Influenza viruses consist of three different major types known as A, B, and C. Of the three types, only A and B cause significant disease in humans. Both A and B types cause the seasonal epidemics around the world (most are associated with type A viral stains), but at any one time there may be hundreds of different variations of each type circulating the globe. Both influenza A and B virus undergo frequent mutations that frustrate the human immune system's attempt to develop long-lasting immunity.
An influenza A virus has two specific surface antigens, hemagglutinin (HA) and neuraminidase (NA), that vary in their chemical structure from year to year as a result of mutation. This genetic drift results in virus particle proteins with subtle variations in structure. Currently, 15 different HA subtypes are known to exist, while there are nine different NA subtypes. These subtypes receive different number designations, and the various influenza strains are named by the specific HA and NA proteins on the virus.
Type A influenza viruses exist in nearly all animals, including domesticated birds and pigs, and these animal viruses bear a close genetic relationship to human influenza viruses. This occasionally allows new virus strains to develop in animals and then leap to humans.
Viruses causing recent global concern include the severe acute respiratory syndrome (SARS) virus, the H5N1 avian flu virus, and a novel type 2009 A (H1N1) virus (initially identified as a "swine flu" virus). Viruses currently circulating in the human population may have common ancestors with viruses associated with recent global pandemics such as the Spanish flu pandemic of 1918–1919. The SARS-CoV-2 virus responsible for the global COVID-19 pandemic is genetically related to the Middle East respiratory syndrome coronavirus (MERS-CoV) and the severe acute respiratory syndrome coronavirus (SARS-CoV, or SARS) responsible for outbreaks of disease emanating from Guangdong Province, China, in 2002 and 2003.
Viruses also differ in their transmissibility: the 2009 A (H1N1) virus is, for example, highly transmissible (similar to a normal seasonal flu), while the more lethal H5N1 avian flu is not yet easily transmissible between humans. SARS-CoV-2 has proved highly transmissible.
First identified in April 2009, the 2009 A (H1N1) virus quickly spread from outbreaks in Mexico and the United States to more than 70 countries around the world within two months. On June 11, 2009, World Health Organization (WHO) officials declared that 2009 H1N1 influenza was a global pandemic.
Because viruses mutate and mix with other viruses, experts attempt to monitor and test viruses for changes that may increase the capacity for human-to-human transmission, or create a more lethal virus that still shows the same ease of human-to-human transmission.
Some viruses show resistance to antiviral drugs.
The 2020 Nobel Prize in medicine or physiology was awarded to researchers who discovered the hepatitis C virus. Their work helped create blood tests for the virus that greatly reduced hepatitis that can cause both liver disease and liver cancer.