Disease is a state of illness resulting from the disruption of an organism’s delicate physiological balance or homeostasis. The disruption results from a disturbance of the normal control mechanisms that regulate the chemical, physical, and physiological processes in the body (Table 10.5). This imbalance or disruption may have its origin in a single event, or it may result from the cumulative effect of a series of events.
Infection is the most common cause of disease. It occurs when a living microorganism enters the mammalian body and causes disease by altering cell structure or function. These microorganisms are called pathogens and include bacteria, viruses, protozoa, and fungi. Infections can be contagious and spread through hand-to-hand contact, airborne particles, food or water contaminated by infectious material, bites from infected animals, or sexual transmission.
Different infectious agents cause markedly different diseases, reflecting the diverse ways they damage cells and the body’s tissues (Fig. 10.5). An ongoing struggle or “horse race” ensues as the pathogen attempts to reproduce and spread, while the immune system tries to eradicate the infection by specific killing of infected cells.
Many medical conditions are incurable, but advances in clinical research keep scientists always looking for true cures. Whether cures mean long-term symptom management or complete eradication, they are essential to maintaining health and quality of life.