Bacteria and viruses cause

Infectious diseases are disorders caused by organisms - such as bacteria, viruses, fungi or parasites. Allergies occur when the immune system reacts to a foreign substance - such as pollen, bee venom or pet dander or a food that doesn't cause a reaction in most people. A disease in which the function or structure of the affected tissues or organs changes for the worse over time. Osteoarthritis, osteoporosis, and Alzheimer disease are examples. Non-communicable diseases (NCDs) are medical conditions or diseases that are not caused by infectious agents. These are chronic diseases of long duration, and generally with slow progression and are the result of a combination of genetic, physiological, environmental and behavioral factors.