More than 2,000 years ago, the Greek doctor Hippocrates began categorizing diseases in a scientific way. This practice continues into modern times. The World Health Organization's current disease classification system can be summarized in 10 broad areas.
Heart, Lung and Other Organ Diseases
The No. 1 killer of Americans, according to the Centers for Disease Control and Prevention, is heart disease. Heart attacks, coronary heart disease and congestive heart failure are all common. Lung diseases, such as chronic obstructive pulmonary disease, also cause many deaths. Causes of these ailments, as well as those of other organs, include inherited conditions, infections and trauma.
Blood and Immune System Diseases
In a 2010 CDC report, anemia was named the leading reason for more than 200,000 emergency room visits and nearly 400,000 hospitalizations. In 2002, the National Institutes of Health noted that about 5 to 8 percent of Americans are affected by an autoimmune disease. Among the most prevalent are type 1 diabetes, rheumatoid arthritis and psoriasis. Immune system problems may be congenital or, as in the case of AIDS, acquired.
The CDC lists cancer in second place as a cause of death in the U.S. Although breast cancer is the most common cancer found in women, and prostate cancer the most common in men, lung cancer is the deadliest. Lung cancer, according to the American Cancer Society, is the top cause of cancer deaths in both sexes.
Injury ranks first as a killer of persons age 1 to 44, according to the CDC. The risk for injury varies with age. For example, poisoning is a particular hazard for toddlers while older adults are at a higher risk of fall-related injuries. Motor vehicle crashes, while decreasing in frequency at the time of publication, continue to threaten all age groups.
Brain and Nervous System Diseases
Brain and nervous system illnesses can appear at any age. Spina bifida, which is associated with inadequate folic acid intake in a pregnant woman's diet, is present at birth. Alzheimer's dementia, like schizophrenia, arises in adults, has a genetic component and is associated with physical changes in the brain.
Endocrine System Diseases
Endocrine glands secrete hormones into the bloodstream with wide-ranging effects. For example, diabetes occurs when insulin from the pancreas can no longer effectively regulate glucose. Cushing's disease of the adrenal glands and hyperthyroidism are also endocrine disorders.
Infectious and Parasitic Diseases
Historically, infectious diseases such as tuberculosis have played a major role in human health. Even today, according to the National Institutes of Health, more people die worldwide of infectious diseases than any other cause. Bacteria, viruses and parasites all contribute to this toll.
Pregnancy and Childbirth-Related Diseases
The number of babies, out of those born alive, who survive to age 1 is a critical health measure. The U.S. infant mortality rate lags behind other industrialized countries. More than one-third of these deaths, according to the CDC, are related to premature birth, which is often related to other problems during pregnancy.
Inherited diseases can be the result of a single gene abnormality -- such as sickle cell disease or Down's syndrome -- or from the interplay of multiple genes. Neural tube defects and hip dysplasia involve multiple genes.
Environmental health effects can be immediate, such as heat wave-related deaths or carbon monoxide poisoning. Others, such as skin cancer, take years to evolve. And others, such DES-related tumors, may only show up in later generations.