The human body possesses sophisticated mechanisms to ward off and recover from disease. Yet, it remains vulnerable. Categories of human diseases, based on the nature of the underlying disorder, help guide research and treatment. Infectious, metabolic, autoimmune and malignant diseases encompass many of the thousands of known human maladies.
Infectious Diseases
Infectious diseases are those illnesses caused by invasion of the body with viruses, bacteria, fungi or parasites, explains the World Health Organization. As the environment teems with potential invaders encountered on a continual basis, the potential for infectious disease proves a constant threat.
The immune system is the body's defense and response mechanism to fight infectious diseases. Nonspecific immune mechanisms patrol the body surfaces. Microbe-specific mechanisms provide targeted immune responses. Illnesses caused by infectious microbes and parasites range from mild annoyances, such as with the common cold or a stomach virus, to imminently life-threatening, such as with toxic shock syndrome or Ebola virus infection.
Metabolic Diseases
Overall body function depends on the intricate balance of hundreds of metabolic processes, which are fine-tuned on an ongoing basis as internal and external body conditions change. Metabolic diseases can occur due to isolated genetic mutations that disrupt specific chemical reactions in the body, as is the case with phenylketonuria and glucose-6-phosphate dehydrogenase deficiency. More commonly, metabolic disorders are acquired.
In the United States, type 2 diabetes mellitus is a leading cause of acquired metabolic disease, affecting nearly 8 percent of the population, according to the National Institute of Diabetes and Digestive and Kidney Diseases. Other common metabolic disorders include osteoporosis, Cushing syndrome and thyroid deficiency and overactivity.
Autoimmune Diseases
The immune system possesses complex mechanisms to distinguish body cells from invading organisms and foreign substances, or "self" from "non-self." As Dr. David Dugdale III explains in a National Institutes of Health Medline Plus entry, autoimmune diseases arise when specific body tissues are mistakenly identified as non-self and attacked with the full fury of the immune system's response to invaders. Cellular destruction and scarring can take a significant toll on the overall function of the targeted tissues. Examples of autoimmune diseases include systemic lupus erythematosus, autoimmune hepatitis, rheumatoid arthritis, myasthenia gravis and pernicious anemia.
Malignant Diseases
Malignant diseases, or cancer, are the second leading cause of death in the United States. The National Cancer Institute notes that more than 100 different forms of cancer can occur, each possessing the common characteristic of uncontrolled cell growth that defines cancer. The American Cancer Society estimates more than 1.5 million new cases of cancer and approximately 569,500 cancer deaths occurring among Americans in 2010. The most common non-skin cancers among American men are prostate, lung and colorectal cancer. Among women, the most prevalent cancer sites are the breast, lung and colon or rectum. Lung cancer remains the leading cancer killer of American men and women. Among children, the most common cancers include leukemia, brain cancer and neuroblastoma.
References
- World Health Organization: Infectious Diseases
- National Institute of Allergy and Infectious Diseases: Common Cold, Cause
- Centers for Disease Control and Prevention: Viral Gastroenteritis
- Medline Plus: Toxic Shock Syndrome
- Centers for Disease Control and Prevention: Questions and Answers about Ebola Hemorrhagic Fever
