In the world as a whole there are three broad groups of diseases that account for a highly significant proportion of illness and death: communicable diseases; degenerative diseases; and neoplastic diseases (cancers). The relative frequency of each of these varies according to the socio-economic state of a country and the condition of its environment. Communicable diseases account for a large proportion of illness and death in developing countries, whereas infant mortality due to communicable diseases is extremely low in the developed world. On the other hand, degenerative diseases (such as those of the heart and circulatory system) and neoplastic diseases (cancer) account for a large proportion of illness and death in developed countries, where a significant proportion of the adult population becomes exposed to environmental factors that produce cancer and degenerative diseases. In the developing world, however, where infant mortality is relatively high, a much smaller proportion of people survive long enough to be exposed to agents capable of producing cancer and degenerative conditions. Even those who survive into adulthood may escape because the agents may not be present in the environment. It should be noted that these diseases are nevertheless widespread in the developing countries, but statistics on mortality and morbidity due to them are rarely available.
A disease or disorder that has more than one identifying feature or symptom is a syndrome.
The health status of the majority of people in the disadvantaged areas of most countries of the world is low. This is shown by the high morbidity and mortality rates that exist in the rural and peri-urban populations that still constitute 80-85% of the population of the world, where some 550 million people suffer from absolute poverty. Although morbidity and mortality show a downward trend, problems such as malnutrition, communicable diseases, parasitic infestations, and others continue to take a heavy toll of people's lives, especially those of infants, children, and other vulnerable groups in the disadvantaged areas. Low health status not only manifests itself in terms of illness and death, but also affects human development and the capacity of individuals to develop their potentialities and lead a productive life.
The health aspects of traffic accidents are of worldwide concern. It is estimated that more than 10 million people are injured on the world's roads each year; there are 250,000 deaths and the incidence of accidents is constantly increasing. The amount of disability that results from the associated morbidity is considerable, bearing in mind that in technically developed countries a substantial proportion of cases of cerebral injury in the community, as well as serious handicaps of a permanent nature, have been caused by road accidents. But the full extent of the morbidity cannot be estimated as not all injuries caused by road traffic accidents are officially recorded.
The worldwide trends in smoking-related mortality and morbidity are alarming. Tobacco-smoking is a major cause of chronic bronchitis, emphysema, and lung cancer, as well as a major risk factor for myocardial infarction, certain pregnancy-related and neonatal disorders, and a number of other serious health problems.
In the developed countries about half of all deaths are due to cardiovascular diseases, a fifth to cancer and a tenth to accidents. These problems are increasing in the developing countries too.
Environmental health problems due to industrialization and urbanization are assuming growing importance; these same problems could affect developing countries as they build up their industries. Chronic disease increases as people grow older. In recent years there has been a steady increase in mental disorders and in social pathology such as alcohol and drug abuse. Lung cancer as well as other chronic lung diseases due to smoking, and obesity due to overeating, are common phenomena.
The significant differences between the population pyramids of the developed and developing worlds are therefore partly explicable in terms of their differing mortality and morbidity patterns.