Viruses are hard to find. An electron microscope, a large and expensive device, is required to "see" them. But before using a microscope, one must first know where to look. Chemical tests of body tissues usually reveal viral chemical activity and, thus, the site of viral infection.
Viruses are responsible for many diseases, such as the common cold, the flu, and some childhood illnesses such as mumps and chickenpox. Smallpox, yellow fever, and certain other deadly diseases are also caused by viruses. In some animals, viruses has been found to cause cancers, such as leukemia in cats. Some cancers in humans may also be caused by viruses.
Viruses are not cells. Cells are the structural units of most living things. Some organisms, like the amoeba, are one-celled organisms. Others, like humans, are multi-celled organisms. Cells contain fluid and specialized structures. Cells reproduce by dividing in half: one cell divides to make two identical cells. Unlike cells, viruses contain no fluid nor do they perform any life processes, such as growth or reproduction, on their own. Instead, viruses infect (live in) the cells of other organisms. By some definitions of life, viruses are not really alive.
Viruses are parasites. A parasite is an organism that lives on or in another organism: called the host. A parasite uses the host's chemicals and nutrients to live and reproduce. Being so small and able to use their hosts' nutrients, viruses survive as very simple physical structures. Usually, a virus consists of a strand or strands of DNA (deoxyribonucleic acid) or a strand or strands of RNA (ribonucleic acid), coated with a layer of protein. Most known viruses have DNA cores. The Human Immunodeficiency Virus (HIV), however, has an RNA core.
[Figure 1]
Figure 1: The Human Immunodeficiency Virus (HIV) - an artist's rendition. Its RNA strands, containing genetic information, are condensed into a cylinder shaped core and coated by two layers of protein. This illustration is not to scale.
In most living things -- animals, plants, microorganisms, and most known viruses -- complete instructions for building and running the organism are contained in DNA. DNA is often called the "master molecule," or the "molecule of life." DNA is composed of four chemical building blocks, called nucleotides, which are like letters of the alphabet. Strung together into different combinations, they form a biological equivalent of words, sentences, and books.
Within human cells, DNA makes RNA. A chemical structure in human cells "reads" a string of DNA and, using a similar chemical alphabet, "writes" a strand of RNA. This process is called transcription, another word for "writing." While DNA stays in the center of the cell, RNA travels around within the cell, building and running the cell, including making other chemicals to do most of the work.
The nucleotide alphabet is basically the same in all living things. A virus uses its host's "alphabetical" chemicals to write copies of itself. To do this, the virus "hijacks" the host cell.
A typical DNA virus first latches onto the outside of a host cell; then it injects its DNA strand into the host cell, leaving its protein coat outside. The virus's DNA travels to the center of the host cell and splices itself into the host's DNA strand or strands. The viral DNA takes over cell operation. The hijacked cell begins to make replicas (copies) of the viral DNA. The host cell also makes proteins for the coat of the virus, creating additional hijackers to invade new host cells. The hijacked cell becomes a virus-making factory. This whole process is called viral replication.
Within a limited number of known viruses RNA not DNA-is the carrier of information. The Human Immunodeficiency Virus (HIV), presumably responsible for AIDS, is a RNA virus. In most RNA viruses, the viral RNA directly hijacks the host cell. However, HIV is different. Once injected into the host cell, HIV's RNA strand writes dual strands of viral DNA (opposite process of human cells). This backwards writing is called "reverse transcription." These newly written DNA strands then go on to hijack the cell and oversee the production of new RNA replicas. Reverse-writing viruses, like HIV, are called retroviruses.
In HIV, an enzyme called reverse-transcriptase (RT) performs the reverse-transcription process. Enzymes are chemical workhorses. Human cells do not contain RT because they only write and have no need to reverse-write. Thus, reverse transcriptase is virus-specific and an important target for antiviral drug therapy.
As a group, retroviruses can live in their hosts for a long period of time without causing any sign of illness. In most animals, retrovirus infections last for life. Retroviruses are not very tough: they die when exposed to heat, are killed by many common disinfectants, and usually do not survive well if the tissue or blood they are in dries up. However, retroviruses have high rates of mutation and, as a result, tend to evolve very quickly into new strains (varieties). HIV seems to share this and other traits with other known retroviruses.
A mutation is a mistake, happening when some nucleotide word or phrase is misspelled. Either a letter or word is left out, or put in the wrong place, or an extra letter or word is added. A mutation is not an intentional act on the part of the organism. Mutants are rarely stronger organisms and many of them simply die.
Suppose this page contained an error in spelling (a mutation). What are the chances that this error would: I) improve the book, 2) make the book worse, or 3) make no difference? In most instances, a mistake makes the book worse. The same is true for organisms. However, a series of small, almost unnoticeable mistakes may lead to the development of a new strain.
Since a single virus may make hundreds of replicas, a few mistakes here and there make little difference in the replication effort. Even if many mutants die, replicas are cheap to make (since its the host cell that pays the price). If one or two of a thousand mutants receive better written instructions for survival in their immediate environment, then these organisms may be able to make replicas of themselves which become, in time, new strains of virus.
For a virus, diversity (having different strains) is an advantage. The new strains may be able to infect (live in) new cells within the host or to infect new host organisms. Many viruses either infect both animals and insects or both plants and insects. Diversity also protects the virus from being wiped out by a host' s immune system. Out of several strains, it is possible that at least one strain will resist an attack by the immune system.
In HIV, a nucleotide segment called the env gene mutates rapidly. Written in the env section are HIV's building instructions for its protein coat. HIV mutants, if they survive, are, likely to have altered protein coats. Since the human immune system recognizes viral invaders by their protein coats, new mutants may be able to hide from the human immune system temporarily, gaining time to replicate and infect more host cells.
The border line between one strain of HIV and another is indistinct. However, from observation in the laboratory, scientists believe they have demonstrated that different strains exist. For example, some strains replicate faster than others. Or, they replicate better in one or another type of human host cell than other strains. These demonstrably different growth rates may explain why some HIV-infected people rapidly grow sick and die while other HIV-infected people survive longer. (Or, a number of other factors may account for this difference in survivorship.) Still, a person who is already infected with HIV should safeguard against being re-infected by other strains.
Two strains of HIV are commonly named HIV- I and HIV-2. HIV- I is the virus first discovered in the U.S., Europe, and Africa. HIV-2 was discovered in Africa several years later. By some reports, HIV-2 causes AIDS and AIDS-like diseases; but by other reports, HIV-2 does not cause illness. HIV-2 may or may not actually be a form of HIV; what is being reported as HIV-2 may actually be two or more different viruses. Mistaken identities are common among viruses. For example, HIV was first thought to be a leukemia virus. Upon discovery, the American researchers initially labeled HIV the "human T-cell leukemia virus-III," or HTLV-III for short. HTLV-I and HTLV-II are cat leukemia viruses. After realizing their mistake, the scientists changed the leukemia in HTLV-III to lymphotropic. (Lymphotropic means "attracted to lymphocytes.")
The presence of different strains complicates the job of detecting the virus in humans. The current clinical (administered in the doctor's office) tests are unable to detect HIV-2. Future modifications may enable them to do so.
During epidemics, the population of viruses greatly increases. As a result, the number of mutants produced increases and more new strains develop. These new strains are likely to cause more trouble in the future, either by being undetectable, or by finding new tissues or hosts to infect. The influenza (flu) virus, an RNA virus, is an example of a rapidly mutating virus which cause humans a lot of trouble. Each winter, new strains return to plague us. Recent evidence suggest that HIV mutates at a rate five times faster than influenza, an extremely rapid rate compared to most known viruses. HIV has the potential to develop many new strains.