The disease process begins when something foreign enters the body, such as a bacterial or viral substance, or when a body system, such as the circulatory, respiratory or digestive system, do not function correctly. Causes of disease include genetics, meaning the inherited genes produce illnesses, and environmental elements that lead to diseases such as cancer.
Digestion
The digestive process occurs from when food or drink enters the mouth to when it exits the body as urine or stool, and this requires many systems and organs to function properly. Digestion for the average adult takes 24 to 72 hours, reports the Mayo Clinic. Diseases that affect the efficiency of the digestive process include Barrett's esophagus, celiac disease, Crohn's disease, diarrhea, hepatitis, irritable bowel syndrome, lactose intolerance, stomach cancer and ulcerative colitis.
Lung
Lungs provide a huge area, as large as a football field, for oxygen to enter the bloodstream and remove the carbon dioxide in the bloodstream from the body, according to the National Women's Health Information Center. Diseases that affect the airway to the lungs include asthma, bronchitis and emphysema. Pulmonary fibrosis and sarcoidosis, or tiny grain-like lumps, are diseases of the lung tissue. Blood vessel clotting or inflammation may cause pulmonary circulation diseases. Other lung diseases include lung cancer, pneumonia and pleural effusion, the collection of excess fluid in the lung tissue.
Ear
The three parts of the ear---the outer, middle and inner ear---provide the channel for sound transmission to the brain. Sound vibrations travel to the eardrum, then to the small bones of the middle ear, and then to the nerve in the inner ear that transmits the nerve impulses to the brain for interpretation. Ear diseases that interfere with the normal transmission of sound waves and lead to potential hearing loss include cholesteatoma, an abnormal skin growth in the middle ear. Chronic middle ear infection, swimmer's ear, Meniere's disease and autoimmune inner ear disease, an inflammatory inner ear condition, are other ear diseases.
Cancer
Cancer, or uncontrolled abnormal cell division, includes the broad categories of carcinoma, sarcoma, leukemia, lymphoma and central nervous system cancers. The National Cancer Institute reports the most common cancer in the United States as nonmelanoma skin cancer with an estimated one million new cases in 2009. Other cancer types include lung with estimated new cases in 2009 at 219,440; female breast at 192,370; prostate at 192,280; colon and rectal at 146,970; and bladder at 70,980. Chemotherapy, radiation therapy, surgery, bone marrow transplantation, and biological therapies are cancer treatments.
