A disease is something that upsets the normal working of any living thing. It can be acute (sudden, but short-lived), chronic (long-lasting), malignant (spreading) or benign (not spreading).
Some diseases are classified by the body part they affect (such as heart disease), or by the body activity they affect (such as respiratory, or breathing, disease).
Heart disease is the most common cause of death in the USA, Europe and also Australia.
Some diseases are classified by their cause. These include the diseases caused by the staphylococcus bacteria – pneumonia is one such disease.
Diseases can be either contagious (passed on by contact) or non-contagious.
Contagious diseases arc caused by germs such as bacteria and viruses (see germs). They include the common cold, polio, flu and measles. Their spread can be controlled by good sanitation and hygiene, and also by vaccination programs.
Non-contagious diseases may be inherited or they may be caused by such things as eating harmful substances, poor nutrition or hygiene, getting old or being injured.
Endemic diseases are diseases that occur in a particular area of the world, such as sleeping sickness in Africa.
Cancer is a disease in which malignant cells multiply abnormally, creating growths called tumors.
In 2009, cancer killed approximately 8 million people worldwide. The risk increases as you get older.