Disease is a disorder of the body or the mind. Diseases affect almost all forms of life, including animals, plants, and one-celled organisms. In people and other animals, diseases are a major cause of suffering, disability, and death. Plant diseases destroy crops, reduce harvests, and ruin gardens. This article discusses human diseases.
Human diseases may occur as sudden illnesses, as long-term disabilities, or as an unavoidable result of aging. All people experience diseases at some time in their lives, but individuals vary greatly in their likelihood of getting any particular disease. This variation is due to many factors, including age, environment, heredity, social conditions, stress, and a person's general health.
Genes are one of the most important factors that influence health. Genes are chemical instructions that control how living things grow, develop, and function. Every human cell contains about 20,000 to 30,000 genes. Genetic instructions are so complicated that many mistakes can occur. Many of these errors can lead to diseases.
Attitudes about health also play an important role in disease. People can make many choices that help maintain health, including not smoking, exercising regularly, and eating a balanced diet. Stress, depression, and other emotional states can influence who becomes ill, how sick they feel, and how quickly they recover.
Ideas about health and sickness differ from one culture to another. In developed countries, many people who fall ill with colds or other minor illnesses stay home from school or work. They may seek a doctor's attention and expect to receive drugs or other treatments. But in less developed nations, sick people may continue to work and meet other demands of daily life unless an illness completely disables them.
Diseases have existed since life began. Scientists have examined 2,000-year-old Egyptian mummies and found evidence of many of the same diseases that trouble people today. But diseases have also changed over time.
For most of human history, infectious diseases, such as plague, smallpox, and pneumonia, were the main cause of death and disability. In some cases, these diseases affected the course of history. For example, many historians think that smallpox was one factor that helped Spanish explorers conquer Indian civilizations of the Americas during the 1500's. Smallpox did not exist in these civilizations until the Spanish unintentionally carried it with them from Europe. The disease swept through the Native American populations, making it easier for the Spanish to subdue them.
The first important advance against infectious diseases occurred in 1796, when British physician Edward Jenner developed the first vaccine
