A disease is defined as a physical or mental disorder or malfunction with a characteristic set of signs or symptoms. Diseases may be caused by a single factor or be multifactorial. There are a number of different causes of disease in humans. Some diseases may be pathogenic, some genetic and some may be caused by lifestyle choices of an individual or people surrounding them.
Pathogens are microorganisms such as bacteria, virus and fungi that cause disease. Pathogens enter our body via various means. Some enter through breakages in the skin while others enter through our respiratory and digestive systems. Pathogens can be airborne e.g. cold virus or waterborne e.g. cholera and can be passed on this way. Each pathogen has a specific method by which it causes disease. When a pathogen enters our body, it may secrete a harmful molecule called a toxin which can damage cells or interfere with the bodily functions (metabolism). For example, tetanus toxins block the functioning of certain nerve cells causing muscle spasms. Other pathogens may damage cells physically by entering the host cell and using up its nutrients to reproduce inside and burst the cell or rupturing the cell membrane.
Cholera is a disease caused by a bacterium, Vibrio cholerae. Transmission is usually via drinking water contaminated with faeces from other cholera sufferers. The bacterium attaches itself to the lining of the small intestine and releases a toxin that binds to specific receptor proteins on epithelial cells of the gut wall which leads to the activation of chloride pumps in the cell membrane. This causes the cell to lose chloride and sodium ions into the small intestine, decreasing the small intestines water potential to below that of the epithelial cells so the cells lose water via osmosis down this concentration gradient resulting in watery diarrhoea. This causes the body to become rapidly dehydrated and some vital ions (sodium, chloride and potassium) very depleted.
Not only can human diseases be caused by pathogens, but they can also be caused by lifestyle choices and environment. For example, emphysema is permanent lung damage caused by smoking and air pollutants (common among miners). The alveolar walls become damaged and replaced with connective tissue resulting in a loss of surface area and thickening of the alveolar walls, making gas exchange less efficient. There is a loss of elasticity. Normal lungs are elastic and will exhale of their own accord, with very little muscular effort needed from the lungs or diaphragm. Emphysema patients have to make more of an effort to breathe out. The main cause of emphysema is smoking. 80% of cases occur in people who smoke and is generally found to affect older people as it is due to cumulative damage over a long period, such as a life time of smoking.