Basic Concepts of Health and Disease
Health is defined by the World Health Organizationas “a state of complete physical, mental, or social well-being and not merely the absence of disease or infirmity.” Health is an ideal state that all of us strive for and may, perhaps, achieve. Most people consider health and disease as two opposite points of a spec-trum and usually place themselves close to the mid-point of this spectrum.
The term disease literally means “absence of ease.” One way of viewing disease is to consider it as a disruption in the mechanisms of the body that maintain homeostasis. The causes of disease are var-ied and different fields of medicine explore different aspects of disease. Pathology is the study of the na-ture of diseases and the changes that diseases pro-duce in the body’s structure and function.
Standard terms are used by health professionals in relation to diseases. The problems described by the patient are symptoms; signs are evidence of disease that can be observed by the health professional. For example, the complaint of pain in a region is a symp-tom; swelling felt in the area is a sign. The cause of the disease is referred to as its etiology. When the ac-tual cause has been identified and named, the disease or condition is said to be diagnosed, or that a diag-nosis has been made. The predicted outcome of thedisease is termed the prognosis. The prognosis of a disease can be good or bad, depending on the history of individuals who have had the disease earlier. Many individuals are predisposed to diseases as a result of factors such as age, sex, race, environment, or genetic makeup. Such factors are called risk factors or pre-disposing factors for disease.
Human disease results from the action of various in-jurious agents on cells and tissue, causing biochemi-cal or structural damage. Disease may be caused by:
· Impaired energy production (e.g., reduced nu-trition or reduced availability of oxygen to tis-sues)
· Impaired immune responses
· Genetic abnormalities inherited from a parent or acquired by radiation, viruses, drugs, or chemicals
· Metabolic toxic agents such as alcohol, drugs, and heavy metals
· Physical agents (e.g., mechanical trauma, heat and cold, radiation injury)
· Chemical agents such as industrial and agricul-tural chemicals and toxic waste
· Infectious agents
· Abnormal development and growth.
Among the various causes of disease, infectious agents gain importance in the clinic, as they can be transmitted from person to person.
Infectious agents are can be grouped into the fol-lowing categories. Arranged in order of structural complexity, they include prions, viruses, rickettsiae, chlamydiae, bacteria, fungi, algae, protozoa, meta-zoa, and Insecta.
Prions are proteins that do not have genetic mate-rial. However, they are infectious and capable of du-plication. Some diseases associated with prions are Creutzfeld-Jakob disease, kuru, scrapie and, in ani-mals, mad cow disease (bovine spongiform en-cephalopathy)
Viruses are the smallest of these agents. They con-tain DNA or RNA strands enclosed in a protein coat and require a living cell for replication and survival. On entering a host cell, the virus directs the nucleus of the cell to function differently and enable viral replication. New viral particles are formed in the host cell and these particles are liberated into the extra-cellular fluid to infect more cells.
Rickettsiae, chlamydiae, and bacteria are simplecells that are small (about 1 micron), with the DNA material enclosed in a cell membrane. They lack a nu-clear membrane. These organisms need specific envi-ronments for survival. The rickettsiae and chlamy-diae, similar to viruses, mainly depend on host cells for survival. Some bacteria are aerobic organisms that require oxygen for energy; others are anaerobic organisms that can survive without oxygen.
Algae, protozoa, and fungi are microorganismsthat have membrane-bound organelles and a nucleus. Algae are organisms that produce oxygen as a prod-uct of photosynthesis. Protozoa are unicellular or-ganisms that may contain flagella. They have a more complex life cycle. Fungi are microorganisms that grow as a mass of branching, interlacing filaments and include molds. Yeasts, which are also forms of fungi, do not have a branched appearance.
Metazoa and Insecta are multicellular parasitesthat affect humans. Metazoa include worms and flukes, and Insecta includes ticks, fleas, and Sarcoptesscabiei (which causes scabies) that transmit or causedisease.
The ability of the infectious agent to cause disease is called pathogenicity. Organisms that readily cause disease are said to be virulent. Organisms that cause disease only when the immunity in a host is low are called opportunistic pathogens. Knowledge of the characteristics of the various organisms helps hu-mans construct strategies to keep infections at bay.
The various routes by which organisms gain entrance to the body are protected by defense mechanisms. If these mechanisms are breached in some way, infec-tion can occur. Once inside the body, the infection may spread directly via the bloodstream or lymphat-ics. One method of containing and preventing infec-tions is to protect these pathways and maintain the barriers. Think of the various ways by which this pro-tection can be achieved as the different pathways of infection are addressed.
Keratinized epithelium, sweat, and sebum are some protective barriers present in skin. When the conti-nuity of skin is breached by laceration, burns, or re-duced blood supply, the barrier is no longer effective, and the risk of infection increases.
Infection may be acquired through the skin by di-rect physical contact (e.g., herpes simplex, ringworm,impetigo), by infection when the skin is disrupted (e.g., tetanus), orby injection into the skin by vectors carrying infectious agents (e.g., malarial parasites in-jected by mosquitoes). Infections may also be ac-quired by injection by humans,such as transfusion of infected blood and blood products and contaminated needles. Certain infections are spread by direct pene-tration of the skin by the infectious agent(e.g., hook-worm larvae).
Infection may also be transmitted by indirect con-tact with infected body fluids via towels, shared uten-sils, or bedding.
Entry of pathogens through the respiratory tract is prevented by the presence of mucus and cilia that move the mucus toward the mouth. Defense cells, an-tibody secretions, and lymphoid tissue (tonsils) in the mouth and pharynx also protect the respiratory tract. Depression of the cough reflex by drugs; interference with ciliary transport, as in alcoholism, cold, and loss of ciliated cells as a result of smoking; and bronchial obstruction as a result of various causes can all con-tribute to the weakening of the barrier and an in-creased risk of infection. Inhalation of droplets car-rying infectious agents is the usual mechanism of transmission.
This tract is protected by lysosomes, antibodies pre-sent in various secretions and the pH of secretions that are not conducive to growth and multiplication of mi-croorganisms. The mucosal lining and the growth of natural intestinal flora in the colon also serve as a pro-tective barrier. Entry of infectious agents through the gastrointestinal tract is via infected food and drink, in-cluding fecal contamination.
This tract is normally sterile. But risk of infection is increased by obstruction of urinary flow, catheteriza-tion, and alteration in normal flora by prolonged use of broad-spectrum antibiotics and others. Entry of infection through this tract is more common in women because of the shortness of the urethra.