A disease is any condition that disrupts the normal functioning of a living organism. It can be caused by pathogenic microorganisms, such as viruses, bacteria and fungi, but also by abnormal proteins known as prions. It can affect any living organism, including humans and animals. It can be infectious (transmitted between individuals via contact) or non-infectious. Disease can be reversible or progressive, and may lead to death.
What determines whether or not a disease exists? In the case of viruses, bacteria and fungi, diseases are often a result of genetic or environmental factors that allow them to infect host cells. However, not all diseases have a clear cause and many that were once considered idiopathic have been linked to certain germs as medical science has progressed.
Diseases are often classified according to their duration, severity and effects on the body. For example, a disease that lasts for more than six months is considered chronic. There are also diseases that are present at birth, which are considered congenital. Diseases can also be the result of a medical treatment, either as an unavoidable side effect or because the treatment was itself ill-advised, and are then called iatrogenic.
Some diseases can be eradicated, while others cannot. Curing a disease involves addressing its underlying causes, managing the symptoms and improving patients’ quality of life. The ability to cure a disease is also affected by a patient’s overall health, genetic predisposition and lifestyle choices.
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