TABLE 1 Factors contributing to population migration in sub-Saharan Africa

1.

Political instability: 75 military coups in 30 countries

2.

Environmental degradation from population pressure, bad land use, weather changes, etc.

3.

Natural disasters such as flooding and earthquakes

4.

Pursuit of productive employment

5.

Artificial boundaries established by colonists, which ignored cultural and ethnic boundaries

6.

Decline in the effect of religion on population migration

Adapted from ref. 9.

population movements, and in particular in reference to HIV infection, no definitive conclusions can be made regarding the origins of HIV; rather, these concepts are presented to provide a better framework to understand the behavioral and demographic changes responsible for the AIDS epidemic.

MIGRATION OF POPULATIONS: DEFINITIONS

Migration is the movement of people in space, often involving a change in the usual place of residence (8). Internal migration is such a movement within national boundaries, whereas international migration involves movement of individuals across political boundaries. Because migration is a continuous, often repeated process rather than a single event, it is often difficult to measure. Migration is further defined in terms of movement in time. The principal distinction is between circulation—i.e., involving repetitive, nonpermanent moves such as daily commuting and other short-term mobility—and definitive migration (8). Periodic movements are mostly short term, whereas seasonal movements have a regular annual rhythm. Long-term circulation involves an absence of more than 1 year but with expectation of return. Definitive migration by contrast implies a permanent movement away from one residence with little indication of return visit. Some examples of internal migration include rural–rural, rural–urban, urban–rural, and urban–urban, each with its own characteristic features and consequences (Table 1).

AFRICA

Following independence in the 1960s, many African countries experienced dramatic demographic changes, which may underlie the movement of HIV infection from potentially remote areas to more populous



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