Human Immunodeficiency Virus

Human Immunodeficiency Virus, infectious agent that causes acquired immunodeficiency syndrome (AIDS), a disease that leaves a person vulnerable to life-threatening infections. Scientists have identified two types of this virus. HIV-1 is the primary cause of AIDS worldwide. HIV-2 is found mostly in West Africa.

HIV belongs to the retrovirus family of viruses, whose members share a unique method of replicating themselves when they infect living cells. Retroviruses store their genetic information in molecules of ribonucleic acid (RNA). However, unlike other RNA viruses, retroviruses use RNA as a template (master pattern) for forming deoxyribonucleic acid (DNA), the genetic material that puts viral replication instructions into effect. This process, called reverse transcription, is the exact opposite of the normal flow of genetic information in living things, in which DNA serves as the template for RNA formation (see Genetics).

HIV consists of a flexible outer membrane, called the envelope, that surrounds a protein case known as the capsid. The envelope is studded with glycoproteins, chemical receptors that enable the virus to lock onto target cells. Inside the capsid reside two identical strands of RNA. These RNA strands make up the virus’s genetic program and store all the instructions needed to replicate HIV once it has infected a host cell. HIV also contains molecules of an enzyme called reverse transcriptase. When HIV infects a cell, reverse transcriptase copies the genetic instructions in the virus’s RNA and uses the instructions to build complementary strands of DNA.

See: How HIV Causes Infections; HIV Treatment

Disease Terminology

Acute - Disease characterized by abrupt or sudden onset, usually with severe symptoms. Acute disease, as a rule, lasts a comparatively short time—no more than a few weeks.

Chronic - Disease characterized by longer duration, often months or years. It is usually associated with symptoms of less severe intensity.

Communicable - Disease that is transmissible by direct or indirect contact with infection.

Complicating - Disease that occurs during or after an illness and has the same cause as the original disease or results from changes produced by the original disease.

Congenital - Disease present in an infant at birth; it may be caused by hereditary factors or result from a prenatal condition or disease.

Contagious - Highly transmissible disease.

Deficiency - Disease resulting from a lack of vitamins or minerals in the diet or a failure to absorb vitamins or minerals from food.

Endemic - Disease that occurs continuously or recurrently in a particular geographic region.

Epidemic - Disease that attacks simultaneously a large number of persons living in a particular geographic region.

Functional - Disease in which there is no significant anatomical change in the tissues or organs to account for the change in function or the performance of the body.

Hereditary - Disease transmitted from parent to offspring genetically.

Idiopathic - Disease in which the cause is unknown.

Occupational - Disease that results directly or indirectly from the patient's job.

Organic - Disease in which there are significant anatomical changes in the tissues or organs.

Pandemic - Disease that occurs more or less over the entire world at the same time.

Primary - Term used in several ways to characterize disease. When an individual has several diseases, the term primary may refer to the initial disease or to the most important disease. Sometimes it is used to denote a disease or group of diseases for which there is no specific cause. At times it is used to indicate the site in which a pathological process begins.

Prognosis - Medical assessment of the probable outcome or the prospect for recovery of the disease.

Psychosomatic - Disease that seems to be caused or worsened by psychological factors. It may or may not produce anatomical changes.

Secondary - Disease that results from a definite contributing factor. For instance, secondary anemia may result from blood loss or blood destruction.

Sporadic - Disease that occurs in isolated cases in a locality where it is neither endemic nor epidemic.

Subacute - Disease characterized by an onset that is not as abrupt as in the acute form and with symptoms less severe and of shorter duration than chronic.

Genetic Disorders

Genetic Disorders, medical conditions caused by an error in a person’s genetic material. Some genetic disorders result in medical problems that are apparent at birth (see Birth Defects), while other genetic disorders do not become evident until childhood or adult life. Genetic disorders range in severity from those that cause death to those that produce only mild problems, such as color blindness. Scientists have identified more than 9,000 genetic disorders. Some of these disorders are extremely rare, while others are comparatively common.

Scientists have identified certain categories of genetic disorders, some of which have characteristic inheritance patterns. One category consists of single-gene disorders—disorders that involve an error in the DNA that makes up an individual gene. A second category of genetic disorders involves abnormalities of chromosomes in which too much or too little chromosomal material is present. Some genetic disorders are said to be multifactorial, because they are caused by the combined effects of multiple genes and environmental factors, such as diet and exposure to certain chemicals. Still other genetic disorders are caused by mutations in mitochondrial DNA.

Currently, there are no permanent cures for genetic disorders, but many treatments are available. A procedure called gene therapy is on the horizon; it may eventually provide permanent cures for at least some genetic disorders.

For a few genetic disorders caused by an enzyme malfunction, it is possible to replace the malfunctioning enzyme with a functioning enzyme. For example, Gaucher disease is an autosomal recessive disorder marked by a shortage of an enzyme called glucocerebrosidase. People with one type of Gaucher disease develop progressive bone disease and an enlarged spleen. They can be treated with enzyme replacement therapy, in which they receive regular intravenous infusions of synthetic enzymes that carry out the functions of glucocerebrosidase.

Most genetic disorders are treated using more than one type of treatment, in keeping with their complex and varied symptoms. For example, children with cystic fibrosis usually take pancreatic enzymes to help digest food and inhale medicines that are formulated to break up mucus in air passages. Parents of these children regularly clap their hand on the child’s chest and back to loosen mucus in the lungs. In some instances, surgeons may perform a lung transplantation to save a patient’s life.

Birth Defects

Birth Defects, also called congenital disorders, any abnormalities of structure or function that are present at birth. The process of fetal development can be disrupted by a variety of external factors such as exposure to radiation, heat, chemicals, infectious agents, or maternal disease. An external agent that alters fetal development is called a teratogen (Greek teratos, “monster”; genes, “born”). Developmental malformations can also be the result of abnormalities in the genetic makeup of the fetus, or they can be caused by a combination of genetic and environmental influences.

Some genetic disorders have symptoms that are manifest at birth. These disorders may result from mutations in a single gene or from more general chromosomal abnormalities. Many diseases and conditions are inherited in a recessive manner: Neither parent may have the defect even though they both carry the causative gene. When both parents have a dominant gene A and a recessive gene a, their offspring may inherit one of four different combinations: AA, Aa, aA, or aa. If the recessive gene a is defective, the statistical probability is that one in four of the offspring will bear the defective trait. In other congenital disorders the presence of only one copy of the recessive gene is sufficient to cause the condition. See also Genetics.

Amniocentesis

Amniocentesis, in medicine, a procedure for withdrawing a sample of the amniotic fluid that envelops the developing human fetus in the uterus. Fetal cells obtained from the fluid are cultured in the laboratory for indications of chromosomal abnormalities, defective enzymes, or deficient growth. Amniocentesis is usually performed during the fourth month of pregnancy in conjunction with genetic counseling of parents with suspected inheritable defects, when the pregnant woman's age might predispose the fetus to genetic abnormalities, or when the woman has been exposed to substances that could seriously harm fetal development.

After injection of a local anesthetic into the abdominal skin, a very fine needle is inserted through the abdomen and the uterine wall into the amnion, the sac that contains the amniotic fluid. About 30 ml (about 1 oz) of fluid is removed by suction for analysis. The procedure has been shown to increase the risk of subsequent miscarriage by 0.5 percent.

More than 75 abnormalities and the sex of the fetus can be ascertained by analyzing the fetal cells in amniotic fluid. Perhaps the most common use of the test is to detect Down's syndrome. See also Birth Defects.