There are nine major causes of disease (a through i below). Frequently a disease may be produced by a combination of these causes, or the same disease may be caused by different factors in different patients, or the cause may be unknown (j below).
a. Prenatal Influences. By this is meant those factors which may operate before birth to produce disease in the offspring; factors may be manifested at birth (congenital disease) or may not become obvious until later in life.
(1) Heredity. Among prenatal factors, one influence is heredity. A disease may be genetically transmitted from a parent to offspring. The parents who transmit the disease to their offspring may or may not have the disease themselves. Examples of some hereditary diseases are hemophilia and congenital dislocation of the hip.
(2) Congenital influence. Diseases affecting the mother while she is pregnant with the baby may adversely affect the offspring. For example, some diseases may be transmitted directly to the baby via the bloodstream, as is often seen in the case of syphilis in the mother. Alternatively, the pregnant woman may have a disease such as German measles, which interferes with the normal development of the child in the uterus (in utero), although, the child does not acquire the disease. Malnutrition in the mother could result in a poorly nourished baby, which could also interfere with the normal development of the child.
(3) Mechanical. Purely mechanical factors are also felt to be responsible for some abnormalities present at birth. Abnormal positioning of the baby in utero is felt to be occasionally responsible for wryneck; torsion or twisting of the umbilical cord would limit the blood and food supply to the baby, and dire results could occur. Any defect or disease present at the time of birth is called a congenital disease or condition. Injuries or effects sustained during the process of being born may be included here.
b. Parasites. Parasites are organisms that live on or within the body of the man or any other living organism, and at the expense of the one parasitized. Parasites may live on the surface of the skin (ectoparasites), or they may enter the body through the skin, the respiratory tract, the gastrointestinal tract, or the genitourinary tract where they may enter the bloodstream and be carried to distant parts of the body. If they live inside the body, but outside the cells, they are called extracellular endoparasites; if they enter the body's cells, they are called intracellular endoparasites. They all cause disease by interfering with the tissue and organ functions; they accomplish this by elaborating toxins, or poisons; by causing inflammation, or irritation; by producing enzymes which destroy tissue; and by causing mechanical blockage of function.
(1) Viruses. These are the smallest agents known to produce disease; whether they are living organisms or complex chemical compounds is not known. They are known to be intracellular endoparasites that cause such common diseases in man as poliomyelitis, common cold, influenza, measles, mumps, chickenpox, smallpox, hepatitis, encephalitis, warts, rabies, yellow fever, and lymphogranuloma venereum.
(2) Rickettsiae. These organisms are larger than viruses, but are still very small intracellular endoparasites. These organisms are transmitted to man by mites, ticks, fleas or lice, and they produce Rocky Mountain spotted fever, typhus (epidemic and endemic), scrub typhus (tsutsugamushi fever), Q fever, and Rickettsialpox.
(3) Bacteria. Bacteria are minute, one-celled, organisms that may occur alone or in large groups called colonies. Significant bacteria can be divided by their shape into three main groups.
(a) Cocci. Cocci are round, one-celled bacteria. The primary members of this group are staphylococci, which group themselves in clusters; streptococci, which arrange themselves in chains; and diplococci, which arrange themselves in pairs. All are pyogenic (produce pus).
(b) Bacilli. Bacilli are rod-shaped; however, they vary from straight to irregular-curved and branched shapes. They cause such common diseases as typhoid fever, diphtheria, tuberculosis, and leprosy.
(c) Spirochetes. Spirochetes are spiral-shaped and can move or twist. Spirilla and Treponema pallidum are examples. The latter causes syphilis.
(4) Fungi. These extracellular endoparasites or ectoparasites are larger and higher in the scale of plant life than are the bacteria. They include the yeast and molds, and produce infections of the skin such as ringworm, and infections of the mucous membranes such as thrush. Some attack internal organs, especially the lungs and central nervous system, very often with disastrous results.
(5) Protozoa. These are one-celled animal parasites (either extracellular or intracellular) that cause such common diseases as malaria and amoebic dysentery.
(6) Metazoa. These many-celled, larger animals include the helminthes (worms) such as the ascaris, the hookworm, the pinworm, the tapeworms, and the flukes, as well as the arthropods (mites, lice, and so forth.).
c. Intoxicants. Intoxication is the process of taking any chemical substance that causes disease or injury into the body. Many substances are very useful in small amounts, and do not cause intoxication; but the same substances may be very toxic in larger amounts, and result in severe illness or death.
d. Trauma. Trauma may be defined as injury sustained by the body as the result of a physical agent or force. The physical agents that may produce trauma or injury of the body are:
(1) Light. In excessive amounts, light can cause temporary blindness.
(2) Heat. Excessive heat can cause burns of the body, heat cramps, heat exhaustion, or heatstroke.
(3) Cold. Cold is absence or deficiency of heat. Exposure to low temperatures can result in frostbite and other cold injury.
(4) Electricity. One can sustain burns, electric shock, or both when exposed to this agent.
(6) Mechanical forces. These agents produce contusions, abrasions, lacerations, fractures, sprains, and strains.
(7) Sound. Exposure to excessive noise can cause temporary or permanent deafness to certain wavelengths.
e. Circulatory Disturbances. Any interference with the blood flow to a portion of the body results in a circulatory disturbance.
(1) Ischemia. A decrease in the normal diameter of an artery supplying a portion of the body results in a decrease in the amount of blood that flows to the part. The area becomes more pale and colder than normal, and is said to be ischemic.
(2) Thrombosis. Whenever a vessel wall becomes diseased, the blood tends to collect at the diseased or injured site and form a thrombus (clot). The presence of an intravascular blood clot is called thrombosis.
(3) Embolism. Portions of a thrombus may break loose, and then travel freely in the bloodstream until stopped by a vessel too small for the particle to pass through; or foreign particles, such as air bubbles or fat globules, may be introduced into the bloodstream and travel freely until stopped by a smaller vessel. These foreign particles are known as emboli. The process of obstruction or occlusion of a blood vessel by a transported foreign material is known as embolism.
(4) Gangrene. When an extremity or portion thereof loses its arterial blood supply as the result of thrombosis, embolism, trauma, or from any other cause, a massive area of the tissue dies, and is said to have undergone gangrene, or to have become gangrenous.
(5) Infarction. Death of the tissue of an organ or portion thereof as the result of the loss of its blood supply is known as infarction. The necrotic (dead) area itself is called an infarct.
(6) Hemorrhage. This is the loss of blood.
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