Disease is a condition or symptom that highlights a disruption of the delicate physiological balance (homeostasis) of chemical, physical and functional processes in the body. The concept of disease is central to medicine and public health. The study of diseases is called pathology and it involves identifying the cause of the disorder or symptoms (etiology), understanding the mechanisms that develop the disease (pathogenesis) and determining the structural changes that occur during the process of disease development.
A pathogen must gain entry into the host body, overcome or bypass innate immune defenses and establish a focus of infection to be considered as having caused disease. Infections are often asymptomatic during the incubation period and may also produce asymptomatic or subclinical infections. Some infections, particularly viruses, have a dormant or latent period (called viral latency) that can extend for long periods of time.
The definition of a disease may change over time, reflecting a mix of social and medical considerations. For example, a new technology for diagnosing a condition may change whether the condition is recognized as a disease. Changing what is considered a disease can have significant social implications such as with the recognition of repetitive stress injury or post-traumatic stress disorder as diseases, which affect the financial and other responsibilities that society has towards those affected. Likewise, the acceptance of aging as a disease has implications for the way in which society views aging and its role in the progression of the human lifespan.