A virus is a small, infectious agent that is made up of a core of genetic material surrounded by a shell of protein. The genetic material (which is responsible for carrying forward hereditary traits from parent cells to offspring) may be either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Viruses are at the borderline between living and nonliving matter. When they infect a host cell, they are able to carry on many life functions, such as metabolism and reproduction. But outside a host cell, they are as inactive as a grain of sand.
Viruses cause disease by infecting a host cell and taking over its biochemical functions. In order to produce new copies of itself, a virus must use the host cell's reproductive "machinery." The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the organism.
Viruses can infect plants, bacteria, and animals. The tobacco mosaic virus, one of the most studied of all viruses, infects tobacco plants. Animal viruses cause a variety of diseases, including AIDS (acquired immuno deficiency syndrome), hepatitis, chicken pox, smallpox, polio, measles, rabies, the common cold, and some forms of cancer.
Viruses that affect bacteria are called bacteriophages, or simply phages (pronounced FAY-jez). Phages are of special importance because
Words to Know
Adult T cell leukemia (ATL): A form of cancer caused by the retro-virus HTLV.
AIDS (acquired immunodeficiency syndrome): A set of life-threatening, opportunistic infections that strike people who are infected with the retrovirus HIV.
Bacteriophage: A virus that infects bacteria.
Capsid: The outer protein coat of a virus.
DNA (deoxyribonucleic acid): Genetic material consisting of a pair of nucleic acid molecules intertwined with each other.
Envelope: The outermost covering of some viruses.
Gene: Unit of heredity contained in the nucleus of cells that is composed of DNA and that carries information for a specific trait.
Host cell: The specific cell that a virus targets and infects.
HIV (human immunodeficiency virus): The retrovirus that causes AIDS.
Human T cell leukemia virus (HTLV): The retrovirus that causes ATL.
Infectious: Relating to a disease that is spread primarily through contact with someone who already has the disease.
Lysogenic cycle: A viral replication cycle in which the virus does not destroy the host cell but coexists within it.
Lytic cycle: A viral replication cycle in which the virus destroys the host cell.
Metabolism: The sum of all the physiological processes by which an organism maintains life.
Orthomyxovirus: Group of viruses that causes influenza in humans and animals.
Proteins: Complex chemical compounds that are essential to the structure and functioning of all living cells.
Retrovirus: A type of virus that contains a pair of single stranded RNA molecules joined to each other.
Reverse transcriptase: An enzyme that makes it possible for a retro-virus to produce DNA from RNA.
Ribonucleic acid (RNA): Genetic material consisting of a single strand of nucleic acid.
they have been studied much more thoroughly than have viruses. In fact, much of what we now know about viruses is based on the study of phages. Although there are both structural and functional differences between the two, they share many characteristics in common.
Structure of viruses
Although viral structure varies considerably among 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. The genetic material in a virus may take one of four forms: a double strand of DNA, a single strand of DNA, 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 100,000 genes that exist within human DNA, viral genes number from 10 to about 200 genes.
Viruses exist in one of three forms, as shown in Figure 1. They are named on the basis of their general shape as rodlike, icosahedral (having 20 sides), or spherical. Some viruses also have an outer covering known as an envelope that surrounds the capsid. The outer surface of some kinds of viral particles contain threadlike "spikes" that are often used in helping a virus invade a host cell (for example, the spherical virus in Figure 1).
A virus remains totally inactive until it attaches itself to and infects a host cell. Once that happens, the virus may follow one of two paths. First, the virus may insert its genetic material (it is always DNA in this case) into the DNA of the host cell. The combined host-viral DNA is then
carried along in the host cell as it lives and reproduces, generation after generation. Viruses that follow this pathway are said to be temperate or lysogenic viruses. At some point in the host cell's life, the viral DNA may be extracted (taken out) from the host DNA and follow the second pathway.
The second pathway available to viruses is called the lytic cycle. In the lytic cycle, the virus first attaches itself to the surface of the host cell. It then makes a hole in the cell membrane and injects its genetic material (DNA or RNA). The viral capsid is left behind outside the cell.
The next step depends on the nature of the viral genetic material, whether that material is single stranded or double stranded DNA or RNA. The end result of any one of the processes is that many additional copies of the viral capsid and the viral genetic material are made. These capsids and genetic material are then assembled into new viral particles. The single collection of genetic material originally injected into the host cell has been used to make dozens or hundreds of new viral particles.
When these particles have been assembled, they burst through the cell membrane. In the act, the host cell is destroyed. The new viral particles are then free to find other host cells and to repeat the process.
Retroviruses make up an unusual group of viruses. Their genetic material consists of two single strands of RNA linked to each other. Retroviruses also contain an essential enzyme known as reverse transcriptase.
The unusual character of retroviruses is that they have evolved a method for manufacturing protein beginning with RNA. In nearly all living organisms, the pattern by which protein is manufactured is as follows: DNA in the cell's nucleus carries directions for the production of new protein. The coded message in DNA molecules is copied into RNA molecules. These RNA molecules then direct the manufacture of new protein. In retroviruses, that process is reversed: viral RNA is used to make new viral DNA. The viral DNA is then incorporated into host cell DNA, where it is used to direct the manufacture of new viral protein.
The first retrovirus discovered was the Rous sarcoma virus (RSV) that infects chickens. It was named after its discoverer, the American pathologist Peyton Rous (1879–1970). Other animal retroviruses are the simian immunodeficiency virus (SIV), which attacks monkeys, and the feline leukemia virus (FELV), which causes feline leukemia in cats. The first human retrovirus was discovered in 1980 by a research team headed by American virologist Robert Gallo (1937– ). Called human T cell leukemia virus (HTLV), this virus causes a form of leukemia (cancer of the blood) called adult T cell leukemia. In 1983–84, another human retro-virus was discovered. This virus, the human immunodeficiency virus (HIV), is responsible for AIDS.
The common cold and influenza
Two of the most common viral diseases known to humans are the common cold and influenza. The common cold, also called acute coryza or upper respiratory infection, is caused by any one of some 200 different viruses, including rhinoviruses, adenoviruses, influenza viruses, para-influenza viruses, syncytial viruses, echoviruses, and coxsackie viruses. Each virus has its own characteristics, including its favored method of transmission and its own gestation (developmental) period. All have been implicated as the agent that causes the runny nose, cough, sore throat, and sneezing that advertise the presence of the common cold. According to experts, more than a half billion colds strike Americans every year, an average of two infections for each man, woman, and child in the United States. In spite of intense efforts on the part of researchers, there are no cures, no preventative treatments, and very few treatments for the common cure.
Viruses that cause the common cold can be transmitted from one person to another by sneezing on the person, shaking hands, or handling an object previously touched by the infected person. Oddly, direct contact with an infected person, as in kissing, is not an efficient way for the virus to spread. In only about 10 percent of contacts between an infected and uninfected person does the latter get the virus.
Contrary to general opinion, walking around in a cold rain will not necessarily cause a cold. Viruses like warm, moist surroundings, so they thrive indoors in the winter. However, being outdoors in cold weather can dehydrate the mucous membranes in the nose and make them more susceptible to infection by a rhinovirus. The viruses that cause colds mutate with regularity. Each time a virus is passed from one person to the next, it may change slightly, so it may not be the virus the first person had.
The common cold differs in several ways from influenza, commonly known as the flu. Cold symptoms develop gradually and are relatively mild. The flu has a sudden onset and has more serious symptoms that usually put the sufferer to bed. The flu lasts about twice as long as the cold. Also, influenza can be fatal, especially to elderly persons. Finally, the number of influenza viruses is more limited than the number of cold viruses, and vaccines are available against certain types of flu.
Influenza. Influenza is a highly contagious illness caused by a group of viruses called the orthomyxoviruses. Infection with these viruses leads to an illness usually characterized by fever, muscle aches, fatigue (tiredness), and upper respiratory obstruction and inflammation. Children and young adults usually recover from influenza within 3 to 7 days with no complications. However, influenza can be a very serious disease among older adults, especially those over 65 with preexisting conditions such as heart disease or lung illnesses. Most hospitalizations and deaths from influenza occur in this age group. Although an influenza vaccine is available,
it does not offer complete protection against the disease. The vaccine has been shown only to limit the complications that may occur due to influenza.
Three types of orthomyxoviruses cause illness in humans and animals: types A, B, and C. Type A causes epidemic influenza, in which large numbers of people become infected during a short period of time. Flu epidemics caused by Type A orthomyxoviruses include the worldwide outbreaks of 1918, 1957, 1968, and 1977. Type A viruses infect both humans and animals and usually originate in Asia, where a large population of ducks and swine incubate the virus and pass it to humans. (Incubate means to provide a suitable environment for growth, in this case within the animals' bodies.) Asia also has a very large human population that provides a fertile ground for viral replication.
Type B influenza viruses are not as common as type A viruses. Type B viruses cause outbreaks of influenza about every two to four years. Type C viruses are the least common type of influenza virus and cause irregular and milder infections.
An important characteristic of all three kinds of influenza viruses is that they frequently mutate. Because they contain only a small amount of genetic material, flu viruses mutate frequently. The result of this frequent mutation is that each flu virus is different, and people who have become immune to one flu virus are not immune to other flu viruses. The ability to mutate frequently, therefore, allows these viruses to cause frequent outbreaks.
The most common complication of influenza is pneumonia, a disease of the lungs. Pneumonia may be viral or bacterial. The viral form of pneumonia that occurs with influenza can be very severe. This form of pneumonia has a high mortality rate. Bacterial pneumonia may develop when bacteria accumulate in the lungs. This type of pneumonia occurs five to ten days after onset of the flu. Because it is bacterial in origin, it can be treated with antibiotics.
Flu is treated with rest and fluids. Maintaining a high fluid intake is important, because fluids increase the flow of respiratory secretions, which may prevent pneumonia. A new antiviral medication is prescribed for people who have initial symptoms of the flu and who are at high risk for complications. This medication does not prevent the illness, but reduces its duration and severity.
A flu vaccine is available that is formulated each year against the current type and strain of flu virus. The vaccine would be most effective in reducing attack rates if it were effective in preventing influenza in schoolchildren. However, in vaccine trials, the vaccine has not been shown to be effective in flu prevention in this age group. In certain populations, particularly the elderly, the vaccine is effective in preventing serious complications of influenza and thus lowers mortality.