In human beings, disease is often used more broadly to refer to any condition that causes discomfort, dysfunction, distress, social problems, or death to the person afflicted, or similar problems for those in contact with the person. The terms disease, disorder, medical condition are often used interchangeably.
Many different factors, intrinsic or extrinsic to a person can cause disease. Examples of intrinsic factors are genetic defects or nutritional deficiencies. An environmental exposure, such as second-hand smoke is an example of an extrinsic factor. Many diseases result from a combination of intrinsic and extrinsic factors.
There are many different factors that can cause disease. These can be broadly categorized into few categories such as social, psychological, chemical, and biological. Some factors may fall into more than one category.
Some diseases such as influenza are contagious and infectious. Infectious diseases can be transmitted by any of a variety of mechanisms, including aerosols produced by coughs and sneezes, by bites of insects or other carriers of the disease, and from contaminated water or food (possibly by faeces or urine in the sewage), etc. Also, there are sexually transmitted diseases. When micro-organisms that cannot be spread from person to person might play a role, some diseases can be prevented with proper nutrition. Other diseases such as cancer and heart disease are not considered to be caused by infection. The same is true of mental diseases.
Living with disease can be very difficult. The identification of a condition as a disease, rather than as simply a variation of human structure or function, can have significant social or economic implications. The controversial recognitions as diseases of post-traumatic stress disorder, also known as "Soldier's heart," "shell shock," and "combat fatigue;" repetitive motion injury or repetitive stress injury (RSI); and Gulf War syndrome has had a number of positive and negative effects on the financial and other responsibilities of governments, corporations and institutions towards individuals, as well as on the individuals themselves.
A condition may be considered to be a disease in some cultures or eras but not in others. Oppositional-defiant disorder, attention-deficit hyperactivity disorder and, increasingly, obesity are conditions considered to be diseases in the United States and Canada today, but were not so-considered decades ago and are not so-considered in some other countries. Lepers were a group of afflicted individuals who were historically shunned and the term "leper" still evokes social stigma. Fear of disease can still be a widespread social phenomena, though not all diseases evoke extreme social stigma.
