Anything that damages the body or how it works can be called a disease. Some diseases are caused by microscopic organisms (germs), others by injury, poisons, or the body itself. Nearly all diseases can be treated by modern MEDICINE to some extent.
The most common infectious diseases are caused by bacteria and viruses. Bacteria are single-celled organisms that live outside our cells. They multiply in body fluids and wounds. Many are killed with drugs called antibiotics. Viruses are microscopic particles with their own genetic material. They live in our cells and take them over, but cannot survive without them.
Genetic diseases, such as cystic fibrosis and muscular dystrophy, occur when a person inherits a particular gene or combination of genes that is harmful. Sometimes the harmful genes are passed on unknowingly by unaffected parents; in other cases the gene changes at conception to become abnormal.
The body has sophisticated defenses for attacking germs, called the immune system, but sometimes the system attacks the wrong targets—people have an allergic reaction when the immune system is triggered by harmless substances like pollen and dust.
There are many different types of cancer, but what they all have in common is that cells start to divide uncontrollably, forming a tumor. The tumor may then prevent part of the body from working normally. This is sometimes caused by agents called carcinogens, such as sunlight or tobacco smoke.
The study and treatment of disease is called medicine. Modern medicine aims to prevent and cure disease where possible. Some diseases, such as diabetes, are incurable, but medical treatment can still relieve symptoms and improve a patient’s quality of life.
Many infectious diseases are prevented by vaccination, which protects the body from specific germs. Screening programs can detect the early stages of diseases such as cancer. A healthy lifestyle can also help prevent conditions such as heart disease.